Generally speaking, you should start using a weightlifting belt when you're lifting as much or more than your bodyweight for lower body exercises, and you're closing in on your bodyweight for exercises that require overhead extension.
This guideline varies based on which types of exercises you're performing, but generally, if you're squatting your bodyweight, you should start using a belt.
Likewise, if you're performing overhead lifts (like an overhead press) with 75% of your bodyweight or more, it's a good idea to use a belt to help with core stabilization.