I heard that unless you go for a 1 rep max, you should not use the belt. It helps with heavy lifting, but at the same time your lower back muscles don't have to work as hard and that hinders their development. Just my two cents.
belt or no belt take it slow and easy with deads and squats. perfect your form. I got too agressive and now I'm nursing something wrong in my lower back. Haven't seen a doctor about it yet.