The simplest way is to record a credit as a negative "payment" amount. If your need for credits is fairly simple, that's a good way to go. It's simple, straightforward, and useful.

OTOH, in many businesses, debits and credits are actually different entities, and need to be handled differently. In that case, you'd just want to build a Credit table, with a very similar structure to the Payment table here. It will make some of your calculations of current balances a little trickier, as you'll have to join the tables. But, if they are different entities, it is best to separate them into different tables.