I just got done doing 100 squats, I'm going to make that my daily routine.

I normally dont work out, but I've decided to work on myself as I've just gone through a tough breakup! I want to put on some muscle mass as I'm very tiny, and I want a nicer bum tbh lol

I normally do a lot of walking so the squats didnt really get tough until I hit 80

Is there anything else I should add into my routine? I need to do something with my arms too, as I'm probably the weakest person in the world lol, I tried doing a plank, but it really hurt my lower back, is that normal? Am I just doing it wrong?