First of all, your sims must be boyfriend/girlfriend, then do heaps of romantic interactions until it says beside the actions: (Female/male sim name) is finding (Female/male sim name) irresistable. When that appears, go to romantic actions and the option to propose should be there (either sim can propose). Once you have proposed, you should be engaged, and then under the romantic options there will be "have private wedding", which makes them say their vows and such, and then they will be married-or you can have a public wedding (I haven't done that yet).