how to gain weight - Exercises for beginners

A

Squat 5x5
Pull-ups 5xFailure
(add weight if "Failure" is becoming more than 12 reps)
Overhead Press 5x5


B

Squat 5x5
Deadlift 1/2/3x5
(your choice; deadlifts can be incredibly taxing, and with exhaustion comes poor form, so be careful; sometimes it's better to do a really heavy load for a single set)
Bench Press 5x5


C

Squat 5x5
Pull-ups 5xFailure
Overhead Press 5x5

Do this sequence every week (maybe Monday, Wednesday, Friday) and steadily increase the weight each session. Once you're making progress, feel free to add in other exercises like dips or more Olympic lifts.