Opening a self-service laundry--also known as a laundromat, although that's a trademark--involves a substantial investment in equipment. You'll need a lot of washers and dryers, of course, plus bill changers, vending machines, rolling carts, chairs and tables.
Once you have the equipment, you need an efficient floor plan so that people can get in, do their laundry and leave with a minimum of stress and hassle. The key is to start on the outside and work your way in.
Put front-loading machines on the perimeter. This includes all dryers and as many high-capacity washers as you can fit on the walls. You want these machines on the outside walls for two reasons.
First, because they open from the front, rather than the top, they can be stacked.
Second, because commercial front-loaders have clear panels on the doors, people can see from across the room whether a load is still turning; they don't need to stand over the machines to monitor them.
Arrange the top-loading washers in parallel rows in the center of the space. Each row must back up to a bulkhead--a short wall about 12 to 18 inches wide that houses the water lines and electrical connections. So arrange the washers in back-to-back rows with a bulkhead between them.
Place work tables at the ends of these rows, for customers to sort and fold their laundry.
Leave enough floor space in all aisles for two rolling carts to stand side by side. One person loading a cart from a machine should not be enough to block an aisle.
Place seating areas near windows, but oriented so that people sitting there can keep an eye on their machines. No one likes washing and drying clothes in a hot and humid self-service laundry. Your floor plan can at least ease the discomfort by allowing customers to get as much natural light as possible. Letting them keep their machines in view also lets them "pounce" as soon as a load is done, freeing up machines for the next customer.
Place bill changers, snack machines, detergent vending machines and other such equipment in a convenient location, readily accessible from all areas of the room, but set apart from seating areas. Don't make customers uncomfortable by forcing them to pull out their wallets in front of people sitting around idly.