Hello!
Today I installed windows 7 on my Mac. And now i'm using it. But the problem is that I can't change back to Mac. I haven't deleted anything, Windows is installed on Bootcamp.
Can somebody tell me what to do next?
If you installed it properly and have MacOS X and Windows 7 on your computer, then hold the option key (the one with "alt" printed on it) while rebooting your computer, and you'll be shown two hard drive images; one with MacOS X and one with Windows.
If you made a mistake and overwrote MacOS X while installing Windows, then it's gone
The same step (option key during reboot) will only show a Windows drive that you can boot from.
If MacOS X is there and you want to always boot into MacOS X except when you hold the option key down, then boot into MacOS X, then in System Preferences choose "Startup Disk". If it is not there then I think you have to start all over again. Post for more help if that is the case.