The real key is starting with consistency rather than intensity, otherwise you're gonna get discouraged or hurt yourself after two or three weeks and just give up on everything. I've been through that myself when I got back into weightlifting after a break - I wanted to do too much too fast, I strained something in my back and didn't touch it for months. This time I learned my lesson: just three sessions a week, nothing but bodyweight, but done properly. The guys above talk about push-ups and squats, and yeah that's effective, but add dips on a chair for your triceps, alternating lunges for your legs, and planks for your core. Start with what you can do without collapsing - if you're just doing 5 push-ups that's already good, you'll build up gradually.
The other important thing is rest days. Between each session you gotta let your muscles recover, otherwise you're just gonna be tired and unmotivated. I alternate: like Monday-Wednesday-Friday. The first couple days are gonna be tough because your body isn't used to it, that's normal, you'll have soreness everywhere. But after like three or four weeks, you're really gonna start feeling the difference and that's when it gets fun.