If you can do 20 good quality push ups, you probably are already at a good physical level, so pick programs difficulty 3 maybe to start, or even 4 ? (better start with 3 for a week or 2 and then see if you feel challenged enough, and then go up if needed, than get burned out). Good luck in your fitness journey!
How many sets of push-ups and how may rep squats ? Recommended by some is 100 pushups in sets a day and 200 squats, also sit-ups are good. To lose fat my experience is to eat a little less.
When out and about look for places to do pull-ups and leg raises.