There are 3 kinds of tofu. You'll want to buy the right kind based on how you plan to serve it.
Soft, or silk, tofu is best served fresh with a little nori paste on top, or in salads. It will crumble very easily, so handle with care. Hard, or firm, tofu is more durable, and will stand up to cooking in stir-fry. There's also a medium firm tofu, which is usually used in cooking but is also ok to eat fresh or in salads.