Instead of doing barbell squats with the bar resting on your shoulders do dumbbell squats by holding the dumbbells at your side. It is easier on the back and feels much more natural. You can even start these without weights doing body weight squats.
As for lunges, start doing them with only your body weight and if you don't feel comfortable going all the way down at first you can short step them. Take a long step and bend your front and back knees as far as you can but stop when you feel like you are loosing balance. While a proper lunge has your front leg parallel to the ground when you are done you can stop at a greater angle until you work up to a proper lunge.
In both cases form is important. For a squat a lot of people 1st motion is to push their knees forward, it should be a sitting motion with the knees still above the front of the foot. When you reach bottom and are ready to push up imagine you are pushing the floor away from you as opposed to pushing yourself up off the floor. Keep your knees above you feet, don't let them go out or in, keep them in a straight vertical line.
The same goes with the lunges, concentrate on form not weight. Keep you knees parallel with your ankles and above you feet. The idea is to use your hamstrings to lower you and your quads to lift you up, again pushing the floor away from you.
Go slow, if you go from no working out to Olympic training you will just hurt yourself.