How much should I increase my weights by?

Try to increase the weight you use each week. If you're on a fat loss phase, you'll probably find you get to a point where you can't increase every week because your calories are lower. 

The amount you can increase depends on you and on the exercise. For example, you could probably increase the weight more on stronger exercises like squats than you can for bicep curls as the muscles used are larger and slower to tire.