Don't try to lift yourself up with momentum or inertia of your body, let the muscles do the work. Hand position is very important in this exercise, you should keep your hands on your ears, not pressing against them, but just holding your hands there. You can accomplish this by pointing your elbows outwardly. Do not grab the back of your head, that puts too much strain on your neck and could cause injury. That will get your abs to grow unevenly like most bodybuilders.
Hand position is the same as for sit ups.
Once again, try to reach your knees, but you probably won't be able to do that. Leg Raises can be done hanging from a bar or on that special leg-raise stand. If your abs are weak, to these with your legs bent at the knees. If your abs are pretty strong, do them with straight legs. And don't just stop at a right angle, go as high as you can.