It's quite similar to how you get married in The Sims 2. First, you must build up a relationship with the Sim you wish to marry. Try lots of flirty or romantic interactions (be careful, some Sims don't like being flirted with if you're not very good friends with them). Once you have a good relationship with the Sim, propose "Going Steady" to become their boyfriend/girlfriend. You can then propose marriage to become engaged. Once you've done all that, you can either choose a "private wedding" by clicking on your fiancee and choosing that option, or you can click on the phone and throw a wedding party. If you choose to have a party, make sure you have purchased the required objects from the buy mode, such as a wedding arch, cake, etc.