Sometimes the most interesting characters to play with are ones that are there to PLAY and not be a bunch of statistics and rules. The main thing to know is what KIND of character you want to play, most systems either have a base for most archetypes OR have the ability to customize openly so any option is there.
Things to remember though are that in level based systems you may not be able to do everything you dreamed of from the start at level one and you have to take that into your character development. Sometimes the path you thought you wanted gets turned on it's head because of how the game plays out and the roles everyone takes in the group and you have to adjust to help balance things. Tabletop games and characters are living things. Even if you have a script in mind be prepared to deviate because things can happen to change what your plans are.
Most systems I've played are d20, d10/d100, or d6, These are the dice that do the most in the system because your success or failure is based on this die, the system is labeled by that die. Other dice offer variations for damage scales usually.
Like people have said, the best way to learn is to play, and in a level based game, at level 1 you generally don't start with much and then you slowly get more. Read the rules beforehand and think like your character and dream of what you might be able to do and what you want to do. These will change as your character grows and develops, bu that's fine. Tabletop RPGs live, so don't get stuck on the rules, they're just guidelines on the path you want to take, not the definitive of what path you will take (most of the time)