It's all about the right foods. To keep the fat off, you basically need to keep the protein up, focus on low-glycemic carbs except post-workout, and get some good fats in your diet. A good rule of thumb is 40% protein, 40% carbs, 20% fats. Eat five to six meals per day including protein (with a little bit of carbs) shakes. To prevent fattening, take the bulk of your carbs late afternoon or early evening (best if its after you train).
Then, it's about proper training. If you are new, the first mistake is overtraining. That means too many exercises (so many guys do variations on variations of the same movement all in one workout and it's counterproductive). That also means don't always try to lift 100% of maximum. With proper technique and slow movements, you get a good result with, say, 80%. If you want to go all out, save the 100% for maybe the last set at 1 or 2 reps. Keep to two to three exercises per body part. Try a two or three day split routine. Get rest days. You will need them. I find that short, heavy workouts are the best for me. That way I can train a body part up to 2 times per week. Don't try to lift more than you are really capable of. You will injure yourself and have to stop training. The overall idea is stress and slightly damage your muscles so they will be forced to grow to compensate, but not to wear them out so they can't even recuperate.
Be patient. I saw my first results on pecs in about 4 months. But I am a hard gainer. Bodybuilding isn't a short term thing. Think in terms of 1 year, 3 years, 5 years. Progress isn't always steady.