Talk to the members of the different units. Some will have a recruiting officer to help explain about their unit and the hobby. Get there early and stay late. See all there is to this hobby before moving on. But most important of all, DO NOT buy any equipment.
Every unit equips themselves differently based upon their impression.
The company commander or recruiting officer should be able to provide you with a list of what you'll need to do their impression. They will also advise you of where to obtain the best equipment for the best prices. Although some will allow individuals from 16 to 18 years with the proper liability wavers or if the parent or guardian is present in the battleline with them. You should be in good physical condition. That's not to mean you have to be a the body of a Greek god, but have good muscle tone.