If you want to enter a cheat code, you will have to buy an action replay. you can get them at, wallmart, toys R us, future shop, etc. If you use the action replay to many times on your DS it may mess up your game. If you do end up getting an action relapy a good site to go to is:
To raise affection on the one you want to marry, give them their favorite presents.Also their is an action replay cheat to raise their heart to red.
Check out Gamefaqs.com and look up your HM. They have lots of free gamefaqs done by players. In HM usually you have to see the heart events to get the girl to like you. They happen at specific times of the week. It takes a lot of time and practice to earn money in HM so keep trying.