“Hey, Keith!” [Comma?]
I turned around and smiled when I saw Erin running towards me waving. ['when I saw' seems clumsy. So does 'towards me waving'. Probably best to break this up into more than one sentence.] Her green eyes sparkled in the sunlight and her black hair bounced around as she ran, glinting every so often. [While there's nothing inherently wrong with this sentence, it's a really clumsy way to sneak in description of a character. There are more subtle ways, generally involving not throwing it all in in a single location. How did he see her running if she was behind him? He must have heard her or been tapped on the back by her or something.]
I waved back and she smiled. She finally reached me and excitedly [Unnecessary adverb; normally a character's speech patterns should convey their state of mind.] said, “Guess what? You know that movie that was totally sold out?” ["Totally" sold out? Does anyone actually speak like this outside a teen drama?]
“The Hunger Games?”
“That’s the one. Well, my friend got two tickets for her and me, ["me and her" sounds more natural] but something came up so she couldn’t go.” [I'm not sure how objective these corrections are, but I thought they gave it a better flow. Partly my personal preference though.]
She paused for a moment to catch her breath, [then] she started speaking again but [more calmly]; “She gave me both tickets and said to bring someone else… So I was wondering…” Why is she being so slow to say it? Doesn't make sense unless she's awkward, which relates to the next, more important point.
Her face turned a light pink and she avoided eye contact by looking at the ground.
“If you’d maybe like to go?” She finished.
Then it was my turn to blush, I thought for a moment then nodded, “Sure.”
[Do real people interact like this? I can't imagine anyone other than very socially inept people or young teens doing this.]
She looked back up and grinned, “Great! It starts at 7:00, do you want to meet at the theater?”
“Yeah, let's do that.”
She nodded and handed me my ticket. Then she waved, “I’ll see you at seven then!” I waved back and she walked off towards home. I did the same so that I could get ready for tonight. [Last sentence didn't need to be there. The next paragraph says so.]
Thirty minutes before seven I finished combing through my sandy brown hair and grabbed my jacket. [Does this sentence accomplish anything other than describing his hair? "At half six I was ready. I grabbed my jacket." would convey the same thing and use half as many words.]
“I’m taking the car, mom!” I yelled as I grabbed the silver car keys and headed to the garage. [Again with the pointless description - "car keys" will suffice.
“Alright, drive safe!” She yelled back.
I hopped into the driver's seat [Possessive not plural] of the blue car ["blue" - more pointless description] and started the engine. Then I drove off to the theater.
Once there, I parked the car and saw Erin leaning against the wall close to the theater entrance. I got out and waved to her, she smiled and waved back. [Not how you use a comma. Either split into "...her. She smiled..." or make it "...her; she smiled..."] [The phrase "she smiled and waved" makes it seem like she's putting on a show. "Smile and wave" conveys something phony.]
I went over and without saying much, we walked into the building.
We got our popcorn, drinks, and snacks then took our seats in the movie. We watched the movie together, and during the sad parts Erin cried a little. I would never admit it to her, but I cried a bit too.
After the movie it had started to rain outside. Lightning flashed and thunder roared. [The description of the rain is a bit heavy-handed and clichéd. Thunder always roars. Lightning always flashes. Be more inventive or leave the sentence out altogether.]
“Hey, Keith?” Erin poked me on the shoulder. [I feel the comma gives it better pacing.]
“Can I get a ride with you? I walked here and it's raining, so…” [Remember; "it's" is "it is", "its" is possessive] [Comma for pacing]
I nodded, “Of course you can, come on.”
I grabbed her hand and led her outside. We got into the car and I drove in the direction of her house.
We talked and laughed, we even quoted the movie that we had just seen.
“Hey? Remember when-”
Erin froze in the middle of her sentence and her eyes widened. I took my eyes off her for a second and looked at the road to see an oncoming car. ["Off her" will suffice]
“Look out!” Erin yelled.
I tried to swerve out of the way but it was too late, our cars collided and I blacked out.
When I came to I felt a numb pain in my head, chest, left arm and leg. It didn’t hurt too bad [maybe 'badly' but character voice is expressed through 'bad' so w/e] because of the shock I was in. I found myself on the grass at the side of the road.
Without moving my head, my eyes came to my car and the one that crashed into us. The car that collided with ours was on the opposite side of the road beat up pretty bad. My car was on the side that we crashed on, and it was on fire.
As I watched my burning car a thought came into my head. Erin could still be in the car! [GASP! THIS IS SO DRAMATIC! The exclamation mark is a bit heavy-handed.]
“Erin…” I choked out.
My eyes wildly looked around until they came to a limp figure about five feet away. I tried to get up, but I wasn’t strong enough. So I dragged myself the five feet and as I got closer I realized that it was Erin.
Her eyes fluttered open and her green eyes were very dull.
“Keith…” She whispered.
She had a gash on the side of her head and a large slice on her chest. Her right leg was obviously broken and there was a gash on her left. [Two gashes? Might want to use a different word if you're not referring to the same wound, but keep it if you like.]
Erin smiled despite how much pain she must have been in. ["...despite the pain." will suffice.]
“You don’t look very good,” she whispered.
I could feel tears stream down my face and when they fell they landed in the pool of blood around her.
She grabbed my hand in a weak grasp.
“Don’t cry, Keith.” She said as tears welled up in her eyes, “You need to be tough.”
“But I…I don’t want to live without you.”
“You may have to, I can feel my life slipping away.”
Hearing that broke my heart in two [This is cheesy], and the tears fell faster.
“No…no you can’t…you can’t leave me…please!” I squeezed her hand and it started to rain again. [The pathetic fallacy is too obvious to be effective.]
She smiled, “The moon really makes your blue eyes shine, Keith. I love you, and I’m sorry but I have to leave.”
Her eyes fluttered shut, and I screamed out, my voice cracking, “No! You can’t leave me! I love you to Erin, you can’t leave!”
My tears blended in with the rain and fell into Erin’s blood mixing around.
I herd sirens and two medics soon came and put Erin and I on Gurneys. They tried to pry my hands from Erin’s and I told them, “Please, let me stay with her.”
“Okay.” [I'm no medic, but I think you ignore the patient's wishes for their own safety at this point. And their separation would increase the poignancy of the situation.]
They put us both in the ambulance and after that, I blacked out. This sentence is clumsy but I don't know why. You should probably rewrite it.
When I woke up, I had iv’s in my arm. My mom, dad, and older sister were around my bed.
“Hey bro, how are you doing?” [This is a bit casual for the situation] My sister asked, but instead of answering I asked my own question, “Erin, is Erin okay?”
My sister shook her head, “I’m sorry Keith…”
Tears started streaming down my face again.
“And you should know that it wasn’t your fault. The ones who crashed into you were drunken idiots.”
My sister added, attempting to cheer me up. [Another pacing comma]
I nodded slowly, only half listening, still crying, I asked, “C-could you guys leave me alone for a second?” [And again]
They nodded and walked out of the room.
I love you Erin. I thought, Rest in peace. I’ll never love anyone else. Love, is so fragile.
A general criticism: "I'll never love anyone else." is an odd sentiment to express after a first date, even if your date dies. If you're showing how he felt at the time, it makes sense, but you would recover from an incident like this eventually. You might not forget it, but "I will never love again" is a very immature response to a tragic event. The blushing at the start, to me, implied that this was the first time they dated. You don't have that degree of attachment to a first date. If they'd been together for a year, two years, I could understand it, but even most people who lose their spouses are eventually able to love again. What you need to decide is how far in the future the narrator said or wrote this. A few weeks later, "I will never love again." makes sense. A few years? Not so much.
That they declared love for one another was a bit cheesy. I can understand the "sunshine and butterflies" feeling, but that's normally less love and more a combination of excitement and lust. Only young minds would call that love. It's not the kind of love that lasts forever, or true love, despite how teens and teen authors characterise it.
I think that you're a young teen, and that more life experience or knowledge of tragic events would be required for you to write this story more convincingly.