Also think of what sort of setting your character/s will be in. Not every character is a magical girl with more bows frills and fancy bits than could be considered sane. As others have said, as most manga involves high school students t-shirt and jeans is a good blace to start. Or if there in a sci-fi manga have fun thinking up some awsome looking space suits, or if it's a fantasy, look at real armour and then just pimp it up a little. If there part of a specific faction mayby that faction has a particular clothing type or symbol on there clothing, Naruto's a good example of that.

Pete