How to put on weight- DESPERATLEY

Hi! I’m 16 and about 5ft6 and am just losing weight no gain. I haven’t gained weight since I was maybe 13? Like i haven’t grown. I can’t remember what my actual weight is but I have the figure of a 7year old like so skinny and I eat more than enough. So I don’t know what to do, I look anorexic and I’m just loosing weight going down bra sizes like always fluctuating. Is it a health problem or ? Like I despise my figure and size because I look alien :( and I don’t know what to do!