How should I get started?

Hello all,

I am 20F at 160 cm and 41 kgs.

I used to weigh more but because of health reasons my appetite decreased a lot and now I have to force myself to eat everything. I also used to do some resistance training but have stopped for a few months. I realise that I still look kinda “fat” or untoned because I don’t have enough muscle mass so I really want to make that a goal.

In terms of food, I don’t know how much I eat in a day, could range from 200-1000 but I feel like having a goal number might help me, my maintajce is probably realyyyy low. Should I be eating in a surplus to gain muscle? Would exercise help me gain some muscle even if I don’t eat really well for a while? I am also thinking of getting like mass gained because then I can get a lot in, in one go.

If anyone has advice in terms of what excercises I should be doing or any tips for me that would really be appreciated!

Thank you!