There's a number of them out there. Take a look at the Starting Strength 20 rep squats wikia page for a good list of the variants. (see Google) There's John McCallum's, and Randal Strossen's, and a number of others.
You can do either "breathing squats" or just regular squats. Breathing squats mean you take a minimum of 3 deep breaths between each rep. You'll probably get a bit lightheaded when you first start doing that.
The basic idea:
Start light. Start with maybe, I dunno, 125 lbs on the bar. Maybe lighter.
Warmup first. Be sure to do these in a rack. Squat 20 times, taking as long as you need between each rep, but don't set the bar down until you hit 20. Next session, add 10 lbs to the bar. Repeat. Do this 2-3x a week.
These will get heavy. They will suck. But you'll learn to work hard, and to build up your willpower.
But make sure you're taking in enough protein and calories. 2400 calories isn't going to cut it on any of the 20rep squat programs. Personally, I'd aim for a gain of AT LEAST a pound of bodyweight per week, 2-3 lbs is probably a better number. Also make sure you're getting plenty of protein in your diet; a minimum of 1g per lb bodyweight. (2.2g/kilo)
Also, if you're not already doing it, I'd suggest getting an omega 3 oil into your diet such as fish oil. It will help control the inflammation and has myriad other benefits.
But yeah, that's about all I can offer if you're going to go down the 20 rep squat path.
If you do decide to do this and it gets too hard... don't let it discourage you from lifting. There are plenty of other programs to switch to like 5/3/1 that work very well too.