Tips for building muscle?

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Right, so I've been doing well, I've lost alot of weight and I'm at a plateau that I believe can only be overcome by introducing a weight training regime to my workouts. I have joined the gym today and I also have two wonderful work buddies (One of which is pretty big in terms of musculature from working out) who are going to teach me the ropes. Have you guys got any tips for really maximising gains and getting the most from a free weights workout?

All tips are extremely useful, be it technique or diet or just what works for you, I really want to be successful and achieve my final goals of an athletic figure!

Let me know what you have to say, I'm looking forward to your mfp sage advice ;)