You need to go to Riften. First go the tavern to see a scene with the priest Maramal(?) (though you propably already have done this), afterwards speak to him, in one topic he mentions that you can have wedding in the temple (don't rememeber what, though). After that wear an Amulet of Mara and speak to a person who likes you (to who you have done a favor), and a new topic comes up that allows you to marry. After that go to to the Riften temple, and a simple ceremony happens.
Here's some more: http://www.uesp.net/wiki/Skyrim:Marriage