you wanna gain mass you gotta eat, you want that mass to be muscle you gotta lift. you don't need any fancy supplements.... yet.

Try to get an idea of your cals and macros. It may be tough but once you figure out how much you're eating you can figure out how much more you need to eat to gain weight. Gallons of milk and hard boiled eggs are you friends.

When i was in your position I started building my home gym, scour craigslist daily for equipment.
It doesn't cost much to get the basics coverd for a decent home gym, a few hundred dollar investment could last you the rest of your life.

Find yourself a bar and some weights and set up a basic routine with deads, squats, various presses, push ups, dips, shrugs, pullups, chin ups, dips and row.

good luck