Allo, allo, how do you do? ;D I'm glad you looked here, especially because Imma start putting up tips...starting...NOW. XD
First off, I know the biggest problem when people RP is deciding who to RP as!
Obviously, no one can play every anime/video game character in the world. No one knows EVERY series, EVERY move, etc, of EVERY known character in the universe. That would be weird. What you need to find is a character compatible to you!
First off, who are your favorite characters?
Second step: Which ones can you describe best?
PICK THEM. DO IT.
Now...how do they speak/act?
This is the hardest part because this is where playing the actual character comes in. However, if you've watched an anime or played a video game and heard the voice, write out a sentence you think the character would say and imagine "that" voice saying it. If it seems off either A. The character is not "alive" inside of your mind or B. That's simply not something that character would say.
Naruto (I love using Naruto for examples) can easily be "heard" when someone types something like this: "HOMEWORK!? WE HAD HOMEWORK!? GAAAAH!"
You could "hear" that, couldn't you? If not, go look up a video of "Naruto" with Uzumaki Naruto's voice in the audio. NOW compare it.
Try it with some other character that you want to RP as! GO AHEAD! You can continue with others until you enjoy one enough to want to stick with it!
FIRST STEP OF RPING?