Keeping track of what you eat and how much makes life easy.
Look at the calories of everything you eat, drink, etc. today and add it up.
Compare that to a calorie calculator to see what you need
The 2 main ways of building muscle are weightlifting (using heavy objects as resistance)
And bodyweight exercises (called calisthenics) that use your own bodyweight as resistance
The best exercises depend on which of these you are doing
Growth w/ lifting comes through heavier weights
Growth w/ calisthenics comes through more reps or more challenging exercises
While gaining muscle you need LOTS of protein. No protein = no growth
.7 - 1g of protein per lb of bodyweight is recommended
If you are overweight, aim for ~100+ grams per day