Buy as many things as possible in Tom Nook's store. Not only will his selection get bigger, but you will get points for every purchase. Get enough purchases to become a member. Keep spending at Tom Nook's store to accumulate PIM points, and rewards at the following levels:

300 PIM points: Nook's Cranny model and store membership
3,000 PIM points: Nook' n Go model and 5% discount
10,000 PIM points: Nookway model and 10% discount
20,000 PIM points: Nookington's model and 20% discount