The main reason most people, especially men, start weight training is to build muscle size. Added muscle mass will make you look stronger and more manly, and it's something that anybody can do no matter what their body type is if they follow certain guidelines. If you set aside an hour to weight train 3 or 4 times a week you can reap the benefits of working out to gain muscle mass.
When you first start weight training, you may very well see strength and muscle size gains very quickly no matter what you do. However, many beginning weight trainers make the mistake of not giving all of their muscles equal training time, instead focusing the majority of their workouts on upper body muscles like the chest and biceps. If you're trying to build muscle you'll want to give your body a balanced, symmetric look. Neglecting leg, back, and triceps weight training and overdeveloping your pecs and biceps won't give you that proportioned look. You need to focus on training every muscle evenly.
To build your muscles evenly and avoid overtraining you should split your weight training routine 2 or 3 ways so you train different muscle groups during different workouts. Splitting your body part workouts 3 ways is optimal for building muscle size and strength because you'll be able to train each muscle with a lot of intensity and get plenty of rest and recovery the next time you train them. Split your upper body workouts by training chest and back one day and arms the other day; or use a push pull routine and do chest, shoulders, and triceps one day with back and biceps trained during another workout. Giving the legs their own workoout will guarantee that you don't neglect training them.
You should also strive for more muscle strength to build muscle mass. As your muscles have to deal with more resistance they'll be forced to grow. If you try to do a little bit more weight or reps per set during every exercise you'll gain strenth and eventually more muscle size. You won't get strength gains at every single workout, but if you train with high intensity you'll get the results you want.
For optimal muscle mass gains, you must start off your weight training workouts with the major compound exercises that require multiple muscle groups to lift the weights. You must, of course, also start your workouts by training the larger muscle groups like the chest and upper back. Do those bench presses and squats to start your workouts and save the smaller muscle groups and isolation exercises for later in the workouts.
If you are trying to gain muscle mass quickly you may want to keep a little extra fat on your body. It's very hard to build muscle and lose fat at the same time, so you may be better off training for muscle size for a few months before getting rid of that extra fat for more definition. This is the way most bodybuilders train, though many of them tend to go to extremes, liking being 50 or more pounds heavier when they don't have a competition upcoming for a few months. Don't try to gain too much weight too fast or you'll just end up with too much body fat that you'll have to get rid of later and you'll look out of shape. Just make sure to consume plenty of protein and enough calories to weight train intensely and you'll build the muscle size you're looking for.
If you weight train wisely and get strength gains without overtraining and overeating, you'll keep your proportions while building muscle mass.