First of all you need think of creative ideas to launch a forum. You must have good knowledge of Photoshop, Dreamweaver. HTML5 etc, Giving attractive names to user according to their expertise, giving them award are also nice ways to keep them engage with forum.
I think this is a cool idea about graphic design forum but if you want to own that forum, first you need to be a graphic design expert on Photoshop, Corel draw, dreamweaver...etc relate to graphics and designing. in cases you will need helps your members on forum.
You must have good knowledge of Photoshop, Dreamweaver. HTML5 etc, Providing attractive titles to user according to their expertise, offering them prize are also nice ways to keep them interact with with community. I think this is a cool idea about graphics community but if you want to own that community, first you need to be a graphics expert on Photoshop.
For graphics design you have to knowledge about photoshop illustrator etc. beside it you have to know about to make banner, logos, httml5, flash these kind of things you should know. this could be helpful for you.
The decade is about graphic reality, everywhere you found out graphic related works. For your work you'll need to know about different type graphics knowledge like Photoshop, illustrator etc. For making forum site you need to know different design, software, communication with people, about web design, Typography etc. These things are really helpful to make your work easy.
Seriously you should have atleast 6 years experience and some wisdom to share, the best thing you can share in your forum that will attract visitors is stratedgies on how to build a graphic design business, find work, and get clients. This what most graphic designers really need help with.