My macbook pro won't boot up. Just the apple icon and time wheel run. Any advice?

you want to check how many threads with same question first, and look in

Mac OS version forum that you have; and the notebook MacBook instead of the workstation forum for Mac Pro (non-laptop)

Not a lot of detail to go by with "won't boot" though.

Isolating Issues in Mac OS

http://support.apple.com/kb/TS1388

https://www.apple.com/support/osx/

https://www.apple.com/support/quickassist/

http://www.apple.com/support/mac101/help/

http://www.apple.com/support/mac101/tour/

https://support.apple.com/kb/TS1367

Assume you know what your OS, how to use the Recovery Mode, and how to repair or do Safe boot and other tasks.

Creating a temporary user to isolate user-specific problems: Isolating an issue by using another user account

Identifying resource hogs and other tips: Using Activity Monitor to read System Memory and determine how much RAM is being used

Starting the computer in "safe mode": Mac OS X: What is Safe Boot, Safe Mode?

To identify potential hardware problems: Apple Hardware Test

General Mac maintenance: Tips to keep your Mac in top form

MacBook Pro
https://discussions.apple.com/community/notebooks/macbook_pro

http://www.apple.com/support/macbookpro