You just have to find a server. After years of dealing with servers I highly suggest you just pay for one that is hosted. It will save you so much time an effort. Some of them are like $10 a month for 20 people or so. Get 5 people to chip in $2 and month and you're all set.
Oh, you don't have to pay for a server. You can run your own for free if you have a spare computer. It's just that it's much cheaper in the long run to have it run on a proper server.
If you pay for a server then you don't have to deal with maintaining the computer. Ideally you want your server up 24/7 so your friends can play whenever. If the server is just on your computer then you have to keep your computer running 24/7. Plus, a server is (probably) going to be much faster and smoother than a computer you would have running.