New to bonzai please help

I would say I have never done a bonsai before than actually say I am new but anyway I want to get a Bursera fagaroides and learn how to grow it as a bonsai. The one i want to buy is taller than I would like but I don't want to hurt it with cutting it wrong can someone tell me what should I do first? Should i simply cut it down to the height i want and then just maintain that height? i dont know much about this subject. I appreciate tips, thanks -Tom