Obviously if you are trying to gain weight, you need to be consuming a safe extra 300 calories in addition to your average calorie maintenance level. This will safely increase your ability to put more calories to muscle building and work. If you overshoot and decide to add 1000 calories, let's say your maximum potential cal maintenance level is 2500 for example, the extra 500 calories will go straight to fat. As you increase muscle gain (at an average of about .25 - .5 pounds a week) your average cal maintenance level should slowly increase, make sure to safely accommodate for this but do not go over your potential maximum level.
Building muscle is about 75% diet. Protein Protein Protein =)
It is up to you if you want to target muscle groups, I would suggest starting with MWF for 1 hour, full body and select workouts to the point of exhaustion. Also you should be doing a similar program to 8-minute abs every single day of the week. I would recommend this video :
This
This is not what has given me the abs I have today, this will get your more general core strength though.
If you are comfortable enough to follow a strict diet plan, I would suggest this one given to me:
This
If you are looking to get that Ryan Reynolds look, this is what you should be doing to bulk up yet stay slim. This is also similar to Taylor Lautner's workout for New Moon, in which he sets a record for the highest shirtless to on screen ratio of all time.
Don't fall for dumb gimmicks such as Muscle Milk etc. Yes I just gave you some instructions, but they are all relative to what you can do and what you currently are doing, also this is just a guide for what you should TRY to stick towards doing.
Best of luck! And I feel the ladies in the 19 community would be looking forwards to some post-muscle gain pictures. I would recommend keeping them begging for a while /flex
Edit: Very important tip, you must be eating SOMETHING every TWO Hours or your body will kick your next food intake into the storage mode resulting in pure fat.