Screen Sharing means that you can have someone else help you with your iMac by being able to see your screen on their Mac and be able to show you how to do things, like open files and use menus with your mouse so you learn different things on your iMac.
It’s probably best if both of you look at the help sectionMac OS X 10.6 Help: Sharing your screen with other computer users and Mac OS X 10.6 Help: Sharing the screen of another computer in order to turn Screen Sharing on. For the easiest way to share screens, you both need to be on the same network, so that means if he’s visiting, it will be easy and rather than directing you on what to do, he can work right on the screen of your iMac.
However, if what’s needed is to be able to work on a Mac on another network at another house – which sounds like what you need – it gets a little more complicated. He’ll need to know the IP address of your Mac and go about connecting to it in a different way. Screen sharing will work this way, but if your IP address changes – the one you entered – then you’ll be disconnected or won’t be able to connect in the first place. And being able to get to that other Mac might require changing settings on your router, too, in order to open “ports” so the computers can talk to each other.
So you may need Apple Remote Desktop, which will allow you to set up software and do other administration work on that other Mac. Other remote administration utilities are JollysFastVNC and LogMeIn, which also let you do other things, like control Windows PCs from your Mac, print and share files, etc.