You do not need to be in great shape to scuba dive. People of all fitness levels can enjoy scuba diving and there is not much strength or physical fitness needed to carry the equipment or to swim through the water.
However, you should be able to swim on the surface and tread water fairly well. Before you learn to scuba dive (or go scuba diving), you will be asked to fill out a medical questionnaire. If you have a health condition which might affect you while diving, it is important to check with a physician to make sure it is safe for you to dive.