If you want to know how to bulk up without taking the illegal route, the recipe is very simple:  hard work and high protein.  You're going to be hitting your gym, and hitting it hard.  And you're going to need the building blocks of that muscle you want to add, so you're going to be taking in as much protein as you can.  Your local gym probably sells multiple products that are high in said protein - everything from powder you mix into your shakes, to pills (legal ones!), to power bars, etc etc etc.  These will be a staple of your diet once you decide to put on muscle mass.

Once you're in the gym, you'll be hitting the free weights.  When the experts tell you how to bulk up, they're unanimous that free weights give you the most bang for your buck.  If you're starting from scratch, you'll want to do lower weight but high reps so as not to strain muscles unused to such a rigorous workout.  Talk to your trainer at the gym to get a rundown of which exercises target which muscles, and follow their advice.  They're professionals, they're probably pretty ripped themselves, and they know the ins-and-outs of each machine.

Once you've take a few sessions to strengthen and improve the flexibility of your muscles, that's when you start adding weight.  Want to know how to bulk up the most?  High weight.  Push as much as you can, and just go for it.  Push yourself to your limit, and only stop when you're completely exhausted.  Target whichever muscle group you're most interested in growing, and get to it.

Say, for example, you want to know how to bulk up your biceps.  The best exercise, then, would be curls.  Start with a lower weight to get an idea of what your body can take, and work with that weight for several weeks.  You'll start seeing muscle definition fairly soon.  At that point, you're ready to go with a higher weight that produces the guns you're looking to show off - maybe at the beach, or at the club, or in your frathouse.  He with the biggest guns usually wins, right?

If you're working out that hard, one thing to be aware of is your caloric intake.  Michael Phelps eats something like 6000 calories a day, or three times what the average person eats.  You likely won't be working as hard as him, though, so monitor your food intake and figure out how much gives you optimal energy as well as aesthetic results.  And it's all about the aesthetic results in the end, right?  Otherwise, why even bother working that hard at something that's not necessarily beneficial to your health?

To that end, you'll want to avoid fatty foods.  Instead of steak, eat a piece of white meat chicken or any kind of fish.  You're gonna have to forego some desserts if you want to do it right, too.  And lay off the beer - no guide on how to bulk up includes Natty Light or Budweiser.  Just use your head and take it seriously to get the results you want.