I'm trying to delete a backup created by itunes, but i can't find the itunes preferences and should i connect my phone while doing this?

Hello,

iTunes Preferences are located in the menu bar under iTunes > Preferences. The menu bar is that bar that spans across the entire width of your desktop at the top of your screen, as seen here:

Once you locate the Preferences menu, you'll delete the backup in iTunes by following the rest of the steps here:

1. Click Devices (the iOS device does not need to be connected).
2. iTunes will show the phone number, IMEI, and serial number of the backed up iPhone when you position your mouse pointer over a backup (iTunes shows only the serial number for iPad and iPod touch). Use this to locate the backup for the iOS device you want to delete. Select the backup you want to remove and click Delete Backup.
3. Confirm you wish to remove the selected backup by clicking Delete Backup.
4. Click OK to close the iTunes Preferences Window.

via: iTunes: About iOS backups

http://support.apple.com/kb/HT4946

and

Mac 101: The menu bar

http://support.apple.com/kb/HT2473

Welcome to Apple Support Communities!

All the best,

Delgadoh