Negatives are the answer to your woes OP.
This works for nearly all movements. There's a concentric and eccentric phase of any movement in which you're going for a contraction, and that's what you're going for because you want hypertrophy, (building muscle), and that means contractions are necessary.
The concentric phase is the phase in which we "explode", think of it as the push in push up, or the pull in pull up; whereas the eccentric phase is the opposite, it's the phase in which we're not directly pushing or pulling against gravity, we're resisting it, and that's the phase you're gonna focus on.
Doing a negative means focusing on eccentric overload to force hypertrophy in less skilled athletes. It's incredibly simple, and you can continue to use this technique even after you can do push ups as a way to push past failure.
Simply get into the push up position with your hands fixed in place about shoulder width, tense up your core like your gonna take a punch, retract your shoulder blades and flex your back muscles, (this is called packing your lats), to protect your shoulders during the movement, and simply lower yourself to the ground slowly as if you're about to do a push up. Exaggerate the control on the way down, really try to do this slowly, and then, if you can't push back up at the bottom, simply allow yourself to slowly and safely land on the ground flat, then get back into the starting position and do it again. You can do this on your knees as well to drop the weight, and actually you probably should start with that so you can get an idea of what your joints can tolerate, as that's gonna be different for everyone.
Eventually, after doing this for a while, you're going to be able to push out of the bottom. When you get there remember to push with genuine force out of the bottom, that's called power, and you'll then be able to complete one full proper rep.
Don't get obsessed with how many you can do, that's a trap, simply go until you hit failure, genuinely push yourself but within reason, and pay attention to your wrists and shoulders during this movement ok? I love the push up, but I destroyed both shoulders doing them incorrectly and far too often in the Army, had to have a very painful surgery to correct the left when I got out and the right is still fucked up. Try to focus on the quality of your reps rather than the number, and you'll quickly find yourself more jacked than the guy who's bragging about doing double your push up count and he can't figure out why he's not growing. Quality of reps matters VASTLY more than number, carry that with you for your entire training career ok? Never forget that part.
You can also do negatives with pull ups and squats, though I'd wait a while on pull ups if you can't do a push up. Squats are easy though, just use a sturdy chair or even a bean bag, put your feet shoulder width apart, and simply lower yourself down to the bag by bending at the hips first, then knees, (important!), and trying to control yourself on the way down nice and slow, and if you can't squat back up, simply sit down or fall on your bean bag, then reset and try again.
Eventually, you'll start to really get the hang of all these movements OP, trust me, but it's gonna take practice. You're gonna fail and hit plateaus, but don't ever get discouraged, and never compare your progress to others, (a common trap), as the only progress that matters is yours. If you're stronger, faster, or more durable tomorrow than today, then your training is going well.
Remember to warm up very well every time, walks are great for this, and focus on range of motion. Take it from someone who used to be pretty damn big; you are capable of doing this and sticking with it, you just have to want it bad enough.
If you ever need advice or even just general information, feel free to ask here or message me man, and good luck bro!
