how do i set up voip?

i have a draytek 2820vn and i want to use skype or sipgate.

Quite seriously, if you don't know then you need to do some reading and find out. It's not as simple as plugging a phone into a BT socket, you have to do some of the setting up yourself.
Sipgate have a pretty good site that explains quite a bit, and they can provide equipment that will plug into your Draytek box.
I've spent over 3 months of trial and error in getting VOIP to work for me, and I'm pleased that I put the time in to find out the finer details as I've ended up with a much better service than I had originally hoped for.
In a nutshell, you need a phone to plug into the router - it has 2 VOIP sockets. I don't use Skype so cannot advise on that, if it would work with the Draytek. Once you have the phone you need a VOIP SIP trunk setting up with a provider like Sipgate, then follow the instructions they give to configure your router.