how do i get ios 5 on my phone

You can't update to iOS 5, you can only update to iOS 6.0.1 (that is the only version that is 'signed' by Apple for the 3GS) - so you will lose the built-in YouTube app and the Maps app will change.

To update, connect the iPhone to your computer's iTunes and copy any purchases off the phone to your computer via File > Transfer Purchases. You may also want to copy photos and any important documents off the phone as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the phone by right-clicking the phone 'Device' on the left-hand side of iTunes and selecting 'Backup'.

Then start the update by selecting the phone on the left-hand side of iTunes, and on the Summary tab on the right-hand side clicking the Check For Updates button

Updating to iOS 5+ : http://support.apple.com/kb/HT4972