Advice from a personal trainer: I'm not going to try to tell you "everything you need to know." But the most important factors are watching your calorie intake versus what you burn. The very simple equation (when you cut out all of the fat, hype, and sales gimmicks) is if you burn more than you take in you will lose weight and vice versa (treating yourself to an ice cream sunday after you worked out so hard is counter productive). People often say you need to do "weight training and cardio" but the fact is that the most efficient use of gym time is to incorporate both together in activities like circuit routines. Number of calories burned in the gym is purely based on your heart rate (intervals between your fat burning zone (zone 3) and your cardio zone (zone 4) is ideal). Invest in a heart rate monitor if you want to have a clue what your getting done (the treadmills are inaccurate and you have to hold on, plus the question begs to be asked: what's your heart rate when you're hitting the weights?). And if you're concerned about risking injury and maximal results invest in a good personal trainer. It is the best and cheapest form of preventive healthcare and is completely overlooked by the vast majority of society. And if your trainer doesn't back up what I just said I recommend you get a better one and/or educating yourself. As a newbie though, you should be able to get results quite easily. The number one factor is consistency; your not going to accomplish it all in one session at the gym, so don't bother trying if it's going to risk you burning out. Enough said! Now get moving!