I paid business expenses using a personal credit card before I had revenue coming in through the business. How can I reimburse myself or pay those expenses?

There are several ways to do paying business expenses from personal funds, this is one I prefer

1. Create a dummy bank account called owners, use write checks (do not print them, they are just a form for entering transactions) on that account to enter and pay the bills/expenses.

2. When you are finished, the balance will be negative, make a deposit for the total amount and in the account block select

for a sole proprietor - owner equity, or better owner equity investment (technically you can not borrow from yourself, if you want to be paid back you use equity drawing for that payment)
for a c- or s-corp  - create a current liability account named due-from=[name] and use that
for a partnership - owner investment OR the liability account like the corporation