Beginner! Please help!

To preface, i’m 15, 115 LBs, 5’9. I was 160 in may of 2025 but dropped a lot of weight. Im just starting going to the gym, like i’ve been once. I dropped weight but now i’m just skinny and i want to build muscle. Any tips on building muscle? I know i should sleep eight hours and drink water and i have a 5 day science-based routine made but im super beginner so i just want some extra tips. Also, I just want to see if anyone has been in the same boat as me with diet. I’ve basically been eating the same things for years and years, I have a bad gag reflex and sensory stuff and whenever i try something, which honestly hasn’t been in a while. I basically just eat chicken breast and milk for protein, grapes, apples, sometimes some chips i like, and that’s basically it. Does anyone have tips for eating healthier/ a good diet for building lean muscle? Thanks!