Most Helpful Guy
Think of a couple guys you know and base a character's mannerisms and personality on one of them or a combination of them. I've written a few female characters that way and even I believed they were legit. But honestly, you're going to be more critical of your writing than anyone else will, so you're probably doing better than you think already. Good luck!
Most Helpful Girl
Just don't think much about it and don't let that hold you back. Write the characters how you want and you can worry about the rest later.