how do I download itunes from my computer to my 3g

Your profile says you have an iPhone 3G so you must have a version of iOS. Already installed on it and it would have iTunes as part of it and that would be whatever iTunes was built into that version of iOS. Your profile says that you have iOS 5. But that cannot be correct as an iPhone 3G can only be upgraded to iOS 4.2.1. Go to settings /general/about on your phone and what does it say under "version"? Also get your model number from there. Place your model number into a google search and see if it is indeed an ip3G or an ip3GS. if it is a3GS rather than a 3G as your profile suggests then you can upgrade the iOS to 6.1.3

First back up you phone by having iTunes open on your computer and connect your phone to the computer with a USB cable and transfer purchases ( read http://support.apple.com/kb/HT1848 ) then do a backup ( see http://support.apple.com/kb/HT1766 ) then update the iOS (read the section update your device using iTunes in http://support.apple.com/kb/HT4623 ). Good luck.

You don't. iTunes on your phone is built into the iOS.