If your a lady then yeah to gain a butt you gotta have some fat.
However putting on muscle is putting on weight, yes you WILL put on weight regardless muscle is mass, simple.
Muscle is denser than fat, so if you keep doing your leg press machine,lunges, hip abductors etc the muscle will kinda make your butt nicely shaped and you will gain weight.
You really have to eat healthy tho and if this means you getting a lil chubby face well it’s better than not gaining your gains.
And btw it’s ok if ur flat fr just take your time if ur a ectomorph it’s gonna take time so be patient.
And since you haven’t gone to a gym before or lifted weights if ur U16 and around age 14-15 join a local gym but don’t over do it a dude at my school legit fainted due to low blood sugar he went to hospital. Take care of your self, eat properly even if it makes you have a fat face. You might want to have muscle of the rest of your body too yk, abs biceps and back because it’s just about balance too.cuz I get you might want a bigger butt for the confidence or whatever but