I guess some writers in bigger cities have mentor's but remember what about the first pioneers of graff they did not have any teachers. SO yes u can do it by yourself
You would learn how to paint just by practicing like anything else you do in life. Practice makes perfect and if you are asking "would you even take up graffiti" well that is a yes or no answer and im guessing if you are posting on this site it is a yes for you.
If you just started writing in a black book you should not go out painting right away. Get your styles,colors,3d's,bombs,tags,pieces,straight letters all down pat first. Then when you are ready i would start with a marker and throw up some tags first. and slowly move your way up in the game.
Peace and Piece