Joe2812
I am not a trainer and have never used one so this advice is just from my own experience. I am not 'big' and never have been but am reasonably strong and fit.
Eating is important, you need the enrgy to do what you want to do once you get to the gym so the right kinds and quantities of carbohydrates are important. Be careful though about 'eating big to get big'. You don't need to be big to be strong and getting big just for the sake of it is a good way of raising your blood pressure. If you have a healthy circulatory system your body should be able to tolerate the short term peaks in blood pressure that will occurr in training however to ask it support an unnatural bulk in the long term can cause problems.
Another important aspect of your training is that you should enjoy it which is one reason I often find myself in conflict with gym instructors, particularly those who struggle to impart a standard of physiology which is lower than you'd have with o-level biology. "Thing is mate, power liftin' dunt really build yer up that much, d'yer know wha a mean ?"
Mix your training, some for strength, some for power and some for endurance BUT, do exercises which you enjoy. Also don't keep it all in the gym, if you take part in something competitive you'll find you extend yourself a lot more without even thinking about it.