I know it may seem annoying simply getting a url as an asnwer, but trust me. There is a lot of good information on this page.
http://codex.wordpress.org/Updating_WordPress
Moderator
Jan Dembowski
(@jdembowski)
Forum Moderator and Brute Squad
Especially pay attention to this part, it could save you some grief later on.
http://codex.wordpress.org/Updating_WordPress#Take_a_Backup
Try deactivating your plugins as well as switching to the Twenty Eleven theme then try to upgrade. This is just to level set your installation and will be only temporary.
Also give the 3.5 Master List a read in case you’ve any of those items to worry about.
http://wordpress.org/support/topic/troubleshooting-wordpress-35-master-list?replies=4
