Bulk up brah

Ok I don’t actually talk like that, I just think it’s funny and completely appropriate in my case. I’m 22 and I’m finally joining a gym to meet my weight/ body goals. All my life I’ve struggled to GAIN weight, overall I’ve always been very underweight but healthy and extremely Active nonetheless. I’m hoping I’m not in a rare position and there others like me out here. We can share tips on what actually works in gaining healthy weight that won’t dissipate