A lot of men out there want to get "big", and this is a particularly common goal among men between the ages of 18 and 35. Most people fail, though, because they do not begin with a good, strong base. Here are some things to keep in mind:
Any sort of resistance exercise will help you get bigger, but you need to focus on your basic core lifts. These would be the bench press, squat, and deadlift. These sort of big, compound movements will help you establish a good strength base for your bodybuilding, and it will also help you grow. Research has proven time and time again that doing large, multiple-joint exercises produce more mass gain than exercises that target very specific muscles.
An example would be the squat vs leg extensions. Many people opt for leg extensions because they are not as grueling as squats, but you will not see the same gains with leg extensions that you will with squats. Squats work out the quadriceps, hamstrings, gluteals, calves, back, and abs, while leg extensions primarily engage the quadriceps.
Research has shown that there are acute increases in testosterone levels after exercise, if the exercise is adequately difficult. This would involve multiple sets of a multiple-joint exercise, like the squat, performed at an intensity that limits you to 10 reps or fewer.
It has been theorized that there may be similar increases in growth hormone due to exercise, but the research is not conclusive.
This is why most bodybuilders stick to the adage that "if you don't squat, you don't grow."
Always train heavy, and always incorporate the 3 powerlifting exercises, which are the bench press, squat, and deadlift. Do 3 or more sets of each particular exercise, and keep the weight heavy, with the reps low.
Without establishing this sort of base strength, you will not be able to see the gains that you are looking for.