You should be comfortable across all areas of tech to be offering a support forum as well as have a clear idea of what is going to make you stand out from the rest of the current support forums out there. Are you going to specialize in just one area of tech e.g computers or are you going to keep it broad? These are all things which should be considered before you decide whether you are going to open your own forum or just be a valuable member on another tech forum.
I think it is a good idea to think before hand in ways you could give incentives and attract members that enjoy giving support. You want to reward people's time and at the same time make it reliable. As much as some people love to help, after a while they will need something motivating them to be back and to feel their time is not being spent in vain and are appreciated. 🙂
If you are a tech expert yourself, maybe there is something in particular that you can help people with and start from there. Sometimes, the best way to get a site like that going is to fill a gap that no one else is filling.
One of the most successful tech forums that I know of was started by someone I know. He found a gap in the market, and he filled it. He sells a service on his site, as well as it being a general techie help forum. 😉 That's worth thinking about.
What are your personal interests? It's important to choose a topic which your passionate a boy and can carry discussions on your forum on well 🙂
I like the idea of tech support though as some members have said, you need to find a unique point of support and work on ways of incenvising staff to help out 🙂