My advice will be a bit different, but there's some preliminary things to understand first:
- Determine the nature of your soreness first:
- Back pump (where there is an uncomfortable tightness in the lower back) is normal and nothing to be concerned about.
- Sharp pain, or even a dull pain that is different from general tightness is a symptom of bad form which needs to be fixed before you consider getting a belt.
- Even small changes to your set up can make a big change in the amount you can lift, and the relative comfort of it.
- A belt does not directly lend support, it merely provides something for you to brace against. It is the bracing that protects your back.
There's no right or wrong answer on when you decide to use a belt, but there is a right and wrong way to use it. Based on studies, you can lift more with a belt, and core strength is similarly engaged both with and without the belt. Eventually, you should wear a belt for the work sets. The important thing is that you learn how to use it properly as a training tool, and don't think of it as a safety net. The truth is, you can get injured wearing a belt and those injuries will be worse mostly because you can lift more than you can without it. You need a strong core either way.
When shopping for a belt, look for these things:
- Same width all the way around (about 3-4").
- 10mm thick should be enough for all but the strongest people (greater than 600 lb squats/deadlifts).
- Single prong or lever belt makes it easier to fasten and remove by yourself.
You will see belts that are wider in the back, but they do not lend any more support. They also tend not to be as thick as belts that are made for performance such as what I described above.
Before buying a belt, make sure of the following things:
- Your lower back is not excessively rounding.
- For Deadlift:
- Your shoulders are in front of the bar
- Your hips are between your shoulders and your knees
- You breath into your spine and brace your back before you pull
- You hold that breath until you are fully upright
- You get a new breath or keep holding the one you stood up with when you put the bar back down. (don't go slow, but keep it controlled)
- For Squat:
- The bar is across your shoulders
- You breath into your spine and brace your back before unracking
- You can catch a new breath when you are fully upright, but keep bracing while the bar is on your back.
- You breath into your spine and brace your back during the squat. The cue "bend the bar" can help with keeping the tightness you need.
The only thing that changes when you add a belt is that you are trying to fill the belt with your breath--you still have a responsibility to brace your back.
Anything you can do to strengthen your core will also help.