Once you get your first foreign fruit (from Mom or a villager), plant the fruit (preferably in a spot where a current tree resides, so you don't risk losing it to a dead spot). Once the tree bares fruit, do the same with that fruit until you have lots of foreign fruit trees. Bells are hard to come by in this game, so having a stash of foreign fruit you can sell every three days will help immensely as you struggle to pay off your house.