And I can see what you mean about starting something physical such as sports to keep me motivated and to actually be enthusiastic, but for myself, Working towards a goal such as lifting a certain weight after working towards it gives me a sense of self accomplishment
That's good if you really feel motivated that's great.
So what are your goals? You need to set that straight from the start.
Secondly, form. Train well, minimise injury, and therefore train harder/smarter and get better results.
Thirdly, eat well. Seriously, you have to eat well, but a LOT of food to grow - and not just crisps and chocolate - I'm talking good wholesome food - in decent quantities.
Fourthly, sleep. Being young dumb and full of **** you'll undoubtedly sleep well, but it's worth mentioning.
Fifthly(lol), compound exercises. Squats, Deadlifts, Bench press. Then, overhead press, chins, dips. None of these should be omitted. If you want to develop more explosive/power Olympic style lifting should be considered - however this is hard to teach yourself - this is where a coach or a specific sporting discipline comes into it.
Sixthly, set yourself realistic goals - there are no cheats, shortcuts or one fix all solutions. It takes a lot of time. Hard work. Potentially tears and blood too. Be patient, but be aware of what you're doing. Don't neglect cardio work either - but don't let it rule your life. A good high intensity session lifting weights will give your body a good work out.
Seventhly, don't be suckered into taking all these "miracle" supps - for the moment, concentrate on learning how to lift, well. It seems you're after size, so hit those medium reps hard - but do not neglect some strength training either - it benefits both your skeletal structure, core and overall ability, but it then enables you to lift bigger weights better - thus growing better.
Eigthly, don't be discouraged. There are times you will feel like **** - and not want to go to the gym. Meet girls, go out for a pint or two. Don't let that stop - have as life! However, don't use excuses to get out of the gym - you either want it, or you don't. It takes dedication - don't screw up your life over the gym though (unless you become a sporting superstar with lots of sponsorship....
).
Lastly, good luck and don't hesitate to ask for advice - bear in mind we will be brutally honest, and we will knock you down to earth if you do not pay attention.