I guess it depends on your specific problems, but my general advice would be to start small. Don't worry about getting a deep squat at first; start shallow. You can also use a chair back or counter to support some of your weight.
Take it slow, and start off preferring more sets of fewer reps. If 4 or 5 reps is all you can do, that's fine!
Work your way up over a period of weeks or months, increasing the depth of the squat and number of reps. If you're using something to support some of your weight, slowly wean yourself off of that. Eventually you'll want to be doing fairly deep squats, 10+ reps per set, while holding some light weights. But it's ok if it takes you months (or even over a year) to get there.
(This is of course not medical advice; you should speak to a doctor who knows your specific situation before starting a new exercise regimen, especially if it's something you're already having trouble with.)