Ok so this post holds a little water.
If you're looking for rapid weight gain you can go the legal but harsher on your body route and take m-drol(it's kinda sorta legal, but still a prohormone and legit) or you can take dbol, either one will put tons of weight on you fast.
After 2 cycles and massive eating frenzy/ training routines I personally gained 60 lbs doing 6 weeks cycles of m-drol and epistane. (I know their ph's but it's all I could get at the time for good reason.) That however was coupled with eating til I either felt like or actually did vomit and lifting until I started seeing spots. Try weight gainer 2200, GNC used to sell it but idk if it's still on the market. It's 2200 calories and tastes like drinking sand paper ground up in oatmeal but if you've got dedication you can handle it. If 2200 and dbol don't put weight on you then nothing will.
First thing's first though, get with some of these older fellas who've had a lot of experience and see if they;ll guide you to correct post cycle therapy (pct) and cycle therapy to protect yourself. I'm that idiot who was young, dumb and full of cum. I had to learn trial by error style cause I thought I knew everything, don't make that mistake.
Any questions, we're here to help.