how do i gain muscle

I’m 5’1, 130-135 lbs, and im young. for a long time my daily calorie intake was 1200 and sometimes below. I’ve lost a lot of weight including muscle without really trying. The only form of exercise I do is walking, but even then, i only walk about 10,000 steps a day.

What should I be eating? I’m losing more weight than I’ve been gaining. I increased my intake to 1400 and try to prioritize eating more protein. any tips? pls help im basically flat now :(