Generally in the UK, unless it specifically says that you pay a monthly fee, you would only pay the interest rate on a credit card if you don't pay off the whole balance in full at the end of every month. The balance will increase each month until you pay it back. Roughly it would be the percentage of the balance as APR, divided by 12. There's also usually a minimum amount you need to pay them otherwise you can get fined around £12.
Getting a credit card would show up on your credit record, but nothing would really happen to it unless if you use it and pay it off as the previous poster said. You don't want to not pay it off or not pay off in full very often as this could lower your credit rating if the card company chooses to report it.
Regarding what happens when you stop being a student, you would have to read the terms & conditions to see if the card is either terminated or upgraded to a normal credit card. Also, it's worth shopping around for a credit card if you're planning on using it. A student credit card is not always the best option, you can get ones with low/no foreign exchange fees and low interest rates, as well as 0% interest for a certain amount of times (which means that you can pay for something, not pay it back in full, and no interest would accrue on it). But credit cards can be dangerous, so don't use one unless you know what you're doing.