To gain muscle you'll need 3 things:
1. Diet
2. Training, progressive resistance loading.
3. Rest
If you're serious about your goals start by reading these articles.
Beginning Weight Training
The Diet
If you want to know anything about training or diet, that site is the first site to check out. There's a very thick layer of bro-science en bs to crawl through in the fitness/bodybuilding communities before you figure out the few people who actually know what they're talking about. Save yourself the trouble.
You can spend a lot of time f'ing around in the gym not really progressing before you figure out you need to start doing some reading about proper training and nutrition, I would save myself the time and start educating yourself now (which means more than asking 'how to slab on muscle' on a penis enlargement forum, or listening to the "bro" advise you'll find on most forums)
A couple of things:
1. Start with a full body routine like Starting Strength, or Lyle's basic barbell routine. (Trust me, those 6 day a week splits you see everyone doing are vastly inferior for a natural beginner/intermediate lifter)
2. Focus on compound lifts: Squats, Bench, Pullups, Rows, Dips, Deadlifts, Military/shoulder press. Forget isolation for now.
3. Eat a caloric surplus, see diet article for kcal/macro nutrient guidelines.
4. Gaining muscle is about progressive loading. This means you'll need to progressively increase weight on the bar in order to keep growing.
5. Take proper rest.
6. Running on self-will will hurt progression most of the time. For example when certain training guidelines say you need to deload after so many weeks, do the f'ing dload. When you're told to do 3 sets of 6-8 reps and not hit failer on every one of them. You don't grind out 6 sets to failure.
7. Use proper form, have an experienced trainer look at your exercise form (especially squats) and read up on how to perform them correctly.
8. Do not read the muscle magazine, any advice from guys representing supplement companies. 95% of supplements are utter garbage, do absolutely nothing. Take your protein powder (find a cheap supplier) and your creatine mono. A multi vitamin, fish oil, calcium/magnesium, maybe vitamin D. That's it. The other stuff is a complete waste of money.
9. Don't follow the training routines of pro bodybuilders, these are completely unfit for the natural athlete who is not already very near or over their genetic limit and using steroids.