During your application for a credit card, you will be told which interest rate applies to your specific credit card. This is an annual percentage that you pay. Interest rates can range anywhere between 9% and 21%, depending on the credit card and lender you choose.
The amount of interest you pay will differ each month. It is entirely determined by how much you spend, how much you repay and when you repay it.
The interest rate only applies to any outstanding amount not paid off by the required due date. Every purchase made during the due date period is classified as interest free; it is only when you have an outstanding balance after the due date that you will then pay interest on the amount left.
Each credit card has its own fees, positives and negatives. It is up to you to research and decide which one suits your requirements. This link here can be used to compare credit cards online.