Western belt sizes are in 2” increments. Generally, you would order a belt size 2” larger that your pant size. If your pant size is 38, then you would order a size 40.
This may differ if you will be adding a trophy buckle that can add 2” +/- to the overall length. In this case a size 38 is probably closer to what you need.
You can also measure a belt that you are using now, assuming you want that same size. Measure from the folded over end (where the buckle is attached) to the hole that you are now using and that will give you the size.