Sushi is not usually made at home in Japan. Sushi bars are nearly ubiquitous there, and the Japanese often feel that only an expert sushi chef can make proper sushi. When they eat sushi at home, they order it. It also doesn't make sense to buy a large selection of fish and other ingredients that have to be eaten that night. However, for special events, making sushi at home can be fun and delicious.
If you decide to use raw fish in your sushi, be very careful where you buy it. You can't use just any raw fish -- look for sushi- or sashimi-grade fish. You may have to check out Japanese markets or ask at a local sushi bar. Regular fish is not handled with the intention of raw preparation, so it is likely to contain bacteria and parasites that can only be removed by cooking. Fresh water fish are not suitable for eating raw.
When you do find the right kind of fish, make sure it's fresh. Fresh fish smells clean, not fishy. If the fish is whole, it should have clear eyes and firm, attached scales. If you're looking at filets, watch out for discoloration and soft spots -- these are all signs of fish that is less than fresh. Some fish, notably salmon, is flash frozen as soon as it is caught. If you don't live near the ocean, you may not be able to find fresh sushi-grade fish. Frozen fish should be completely thawed in the refrigerator before use.