I guess you mean in the Dragon Age: Origins game. I found this buggy to begin with. Make sure youre a Human male from the Noble origin, or you can't marry her. When you get the chance to talk to her about the landsmeet in her room at Arl Eamons estate, right after you saved her from Arl Howe, you will get the option. I'm not sure if it can be peformed in another way, but i first asked her about marrying Alistair, i agreed with her that he would be a bad king, and thereafter i could ask her to marry me, i think the dialoug is something like "A stronger 'something.'" The part about making her agree depends on your persuation level, i had mine full, plus 25+ in cunning. The part that was glitchy for me was at the direct landsmeet. When you have won over Loghain, i assume that you musten't kill him yourself, i had Alistair duel him and kill him, but i guess it'll work if you spare him too. After he is dead Eamon and the other houses will turn on you to decide who's to be ruler. Here i messed up twice. Both times i made Queen anora singlequeen. I guess it messes up the game, 'caus she still talked about our wedding which never would happen. In the landsmeet dialouge where you choose ruler you must select the lowest option, wich says something like "I'll to be king by anoras side" and then Anora holds a little speech about your duties. From there you can only mess it up if you back out when she asks about it before hednin' to Redcliffe.
Hope this helps.