Marry a villager, or attend the wedding of another Hero.
You'll need to find someone that you want to marry and then make them fall in love with you by repeating fun/flirty expressions on them. Once they have fallen in love with you a big red heart icon will appear above their heads and you'll be able to propose by giving them a ring as a gift. Once you are engaged go to a house that you own and select 'set as marital home' and you'll be married. If you want to find an easier bride then you can make a whore(in Bloodstone or Bowerstone Old Town) fall in love with you if you're not to bothered about who you're wife is!