If you want to start at the very bottom, do 10 pushups, 10 situps, 10 air squats, 5x a day. Gradually move those numbers up to 12, 15, 20. Google body weight exercises and start mixing in the ones you like. In no time at all, you'll have a nice little routine going.
I think I mentioned it upthread, but my current routine is next to nothing, but it's helped a ton.
60 second Yogi Squat - I've learned to love these. After you get them down, you can really feel all the different muscles, and especially your back posture
60 second plank
20 pushups - I've started to change hand positions each round just to make sure I'm getting all areas of chest
20 air squats - I try and focus on dropping my ass directly over my heels and using my glutes to stand back up as opposed to my quads
Like, that's pretty much nothing, but 5x a day adds up. Honestly, my chest hasn't looked this good since I was lifting regularly 20+ years ago. Not huge or anything, but at least somewhat defined now. Triceps are popping too - I really need to start doing some chin ups for my biceps.