From what I remember about playing this on the GameCube (can't imagine different consoles will be vastly different) was that you had to just keep doing all the jobs that Tom Nook asks you to do and go and talk to people in the town who will also ask you to do jobs, eventually Tom Nook will say he has no more work for you and you will have paid off some money off the mortgage on your house. You are then free to go and get Bells from elsewhere and pay off the mortgage slowly and that means you will be able to buy and sell items in Tom Nooks Shop