How to tone my body and gain weight :)

I recently lost over 100 pounds in quit an unhealthy way. I’m 19 so I do have extra skin but I feel like it’s not so much that if I toned my body my stomach and stuff would be a little bit tighter. I’ve also found that my tits and ass are like flat and flabby now, makes me sad cause I was soooo thick and I loved my body especially my ass and tits. I really want to tone my body but I also want to gain a little bit of healthy weight as well through protein and things like that. Any advice would be great or what did/didn’t work for y’all.