ikristiina:
Sorry to hear what you did. It's very possible your computer's security has been breached, or it might be only your iTunes account. Someone might be stealing stuff right now. So, call Apple Support immediately to report what you did! They should be able to tell you want to do, how to fix it. You might need to make an appointment at your nearest Apple store for further help in searching for hidden programs too.
If you clicked on the link and followed whatever instructions where there, you may have inadvertently downloaded malware, spyware and/or a key logging program that runs in the background. It may be transmitting your every key stroke to someone else now, so all your passwords could be compromised. Or, it may be you only gave up your iTunes account info, so you now need to change your iTunes password. At the very worst, if you did download spyware or a key logging program, then all your other passwords will need changing too, but remember, if you have a key logging or spyware program hidden on your computer somewhere, that program will be transmitting every new password you enter too, so don't take this event lightly!
Do you have your Apple Firewall turned on?
Do you have any virus protection programs on your computer, beyond what it came with?
If not, I highly recommend the ones Intego puts out. They are specifically written for Apple products only. I've been using their VirusBarrier X6 for 3 years now and been trouble free since. I bought the family plan for my daughters' computers too. They have newer versions out now. Get one or more of those.