computeropponen: There are two ways: you can go to a player's page and invite that person directly, or you can make an open invitation to players in general here at BrainKing (with some restrictions if you want to).

For the general invitation, on the left side of the page near the top there is a category called "new game". Click it.

Then select the type of game you want to play, then pick the parameters that you want. Be careful on time controls and such so that you get what you want; there are default settings but I suspect you won't want to use those.