how can i partition my disk as guid?
You didn't say what all is on your disk at the moment. Or, if you have more than one drive to work with. Did you use a patched distribution or Retail DVD to install your system. All of that makes a difference to what the answer is.
If it's just OSX, you have an extra drive and you used retail or the correct distribution then it is rather easy, but time consuming.
You would need enough free space on a second driove before you start. Boot your system from your installer DVD. Select the Utilities -> Disk Utility and create backup images of the partitions currently on the drive. When you have the backup images reboot with the installer DVD. Again, go to Disk Utility. Check to see that you could setup and start a restore operation for your OSX partition. The reason for this is that Disk Utility on some distributions has a bug that prevents getting the restore operation setup correctly.
If everything looks good, use Disk Utility to repartition your drive. On the panel where you set the partition names and sizes select Options -> GUID and then apply the change. Then, use Disk Utility to restore your OSX backup image.
You will also need a copy of the Chameleon boot, boot0 and boot1h files where you can access them from the installer DVD -> Utilities -> Terminal session. You need to manually install Chameleon using the fdisk, cp and dd commands. This is required to make the system bootable after the restore.
See, simple - and time consuming.
Good luck.