first I assume you have a sled right?
Start small like pipes, or clutching, or a head, maybe change some suspension parts etc. Do this your self and you will get more confident about your abilities. You will be able to see first hand what combos work and what did not, and when it does not work right you will change on peice of the puzzle and try again. You will eventually start to get a feel for the better combos after enough wrong turns, especially in the cluthing department. Then you can move on to bigger mods motor tear downs and rebuilds, big bores, twin pipes, nitrous, turbos, you get the idea.
Also hang around people you know who are more savey than you. It will rub off on you.
Working for a dealer will give you experiance on the machines them selves but more than likely not in the mod section, but a good way to learn how to work on them.