With credit cards, am I always charged interest, even if I pay on time?

Regardless of the interest rate they're charging, if you pay off the entire balance that shows on your monthly bill before the due date you will not be charged interest. The interest rate only matters if you carry a balance (i.e. pay less than the total balance showing on your monthly statement). I think the clock on the 6 months zero interest starts when the account is opened, not the first time you charge something, but you should check the terms of your card to find out the right answer specific to this account. If you do carry a balance, that zero percent interest is going to last for 6 months from the starting date - it's not specific to each purchase. You can ring up and pay off however many times you like; if the balance of your account is zero when that 6 month deadline hits, you won't pay interest (assuming all payments are on-time). If you don't pay off the entire balance to zero, interest will start being charged to whatever amount is still showing as your balance at 13.99% interest.

After 6 months you will pay 13.99%.

As long as you pay off your card each month you'll never owe interest. They start counting the 6 month 0 APR when you start carrying balances. Edit: This was the case with my Bank of America card. I owned the card for years and years before I carried balances, and they still gave me 6 months free interest.