>>181 (OP)
simple, same with all of fitness, slow progression, do wall pushups, when those are too easy>domore> increase angle, like push ups against a rail> increase angle, like against a bed (bed push ups are actually hard cause it's soft and good for stabilizer muscles too)
then lower the angle till you are on the floor doing them, then up the reps, then do archer push ups, then do 1 arm push ups, then do pike push ups (more shoulder based) then do against wall pikes, then do handstand push ups.
same thing for all of excercises it's actually pretty simple.
squats, lower squats, squats using mostly one leg power, pistol squats.
pull ups, do pulls on a desk or something with you under, more like rows, jump then fight the fall on a bar, fuck cardio personally, but sprints are good for leg strength.
biceps, use backpack, or lean back and do sorta like pull ups but using biceps.
If you do buy equipment, a pullup bar or make one, and a kettlebell is my reccomendations, simply for the swing and overhead press.
fitness is pretty simple, just slow progression and consistency, but also your muscles gain memory so you don't lose much once you gain it, although going back to fit from not fit usually has an adjustment period where you'll think you lost it all but it's just mainly your CNS central nervous system readapting to taking on loads.
you don't have to go to failure ever, but I prefer it for fun and then just resting a lot.
one thing I disagree with in fitness general discussion is always be working out and how frequent the typical schedule is, I agree with it for light workouts, but I don't like to waste my time, so I do medium to hard full body days and then just chillax and let my body recover, I don't work out when tired or sore, and I am insanely strong, although lots of that are my elite White genetics probably.
good luck fren.