There are certain requirements that must be fulfilled if you wish to marry one of the available bachelorettes or bachelors.
There are also special bachelorettes and bachelors that have much more requirements and an invisible heart level, making them much harder to marry.
In order to marry a bachelor or bachelorette, you must increase their heart level to red first. The heart level will be shown each time you talk to the person, and it will go from black, to purple, blue, green, yellow, orange, and finally, red. You can increase this daily by giving them gifts and talking to them, but just having a red heart isn't enough!
Each girl or guy has four heart events that you will need to trigger. They occur at specific times and locations; you may stumble upon some accidentally. The higher heart events will require your bachelor or bachelorette to be at a certain heart level, or else you won't be able to trigger it even if you show up at the right place and time.
You must upgrade your house to the largest size. This can be done by speaking to Gotz, the carpenter and lumberjack of Mineral Town who lives south of your farm in the mountains.
You will also need to purchase the big bed from the Shopping Network, so your spouse will have a place to sleep when they move in with you.
Finally, you will need to purchase the Blue Feather from the Supermarket. This item becomes available for purchase when your special lady or man's heart level is red. It equips like a tool, and you will use it standing next to or in front of them, and this acts as a proposal for marriage. If you complete all the other requirements, they should accept your proposal and you will get married 7 days later. Congratulations!
If they decline, you have not completed all the requirements.
30 days (or one season) after you and your spouse are married, your wife (or you, if you are playing MFoMT) will not be feeling well one morning. You will go to the clinic, and the Doctor will announce that your wife/you are pregnant.
Your wife/you will give birth in 60 days, and you will have a baby. For the first 60 days, it will be sleeping in the bed and you cannot interact with him or her, but after those 60 days, your baby will grow into a toddler, and you can talk and give gifts to him or her.
In Harvest Moon: Friends of Mineral Town, your child can only be a son. In Harvest Moon: More Friends of Mineral Town, the odds of having a boy or girl are split 50/50, and there are many new features. Your child will grow past toddler stage in this game, unlike the previous one.
50th Anniversary Gift
After you have been married for 50 in-game years, your spouse gives you the Mountain Cottage as an anniversary present.