Hi guys. I am an eighteen year old weighing in at 120 pounds. I will be signing up for a gym tomorrow and I will be working out everyday. The problem is that I don't have a plan. Can anybody give me any pointers and tips on what to do/how to eat?
In short, to gain weight and muscle you need to eat a caloric surplus.
As for the exercises, big compound movements like Squat, Deadlift, Bench Press, Rows, Overhead Press, Chin Ups and dips.
If you've never done any weight training before, do bodyweight exercises (push ups, dips, chin ups etc.) to build a bit of strength before you start throwing some steel around. Get someone to help you learn correct form in order to avoid injury.
Avoid bogus high repetition drills or meaningless circuits and instead focus on lifting heavy and hard in the 6-12 rep range. Also you don't need to workout everyday, and I'd definitely recommend against it, thats an easy way to injure yourself or burnout quickly.
But diet is the biggest thing, and make sure you read all 4 of those threads.