I spent$100 and my APR is 18.99%, How much am I going to owe?

-18.99 interest per yearI am assuming that by "spent", you also mean "borrowed". With your apr, you will be paying $18.99 over the course of the year, assuming the balance stays the same. If you make monthly payments greater than the interest payment, the interest will only be charged on the reduced balance the next month. For example, if you make a $10 payment on the loan the first month, $1.58 will go towards the interest payment (1/12th of the APR), and $8.42 will go towards reducing the principal balance. The next month you will only pay interest on the remaining $91.58 you owe, so the amount of interest goes down.
If you are talking about a credit card and you spent that money in the past 30 days, you'll owe $100 with no interest IF you pay in full before the payment due date.
Probably $450...