Guys, I have some what of a problem, a lot of people are asking me to be a GM on some rol game campaigns, but I have no experience as a GM, only as a player (and a very ridiculous player). So I want to know what should I do as a GM? Like, some tips or advices you guys can give me.

I have been asked for 2 games, Dark Heresy and Nephilim, I dominate both pretty good (probably nephilim a little more), and I can get some campaigns for Dark Heresy, but there are no campaigns of examples of campaigns for Nephilim, so I don't think I can get any info for this (but I wrote part of a campaign for this one so I think I can make it work). So advices?

Oh ya, also, which is the appropriate number of players per campaign?