The mike is the critical part of a sound system, affecting both sound quality and freedom of movement. As a result, many performing storytellers start their investment in sound equipment by purchasing a microphone, which they bring to performances and connect to the existing sound-system at the site.
For most storytellers, the ideal type of microphone is a wireless clip-on.
Clip-on means the element (the end of the mike you talk into) clips to your clothing. (Sometimes, clip-on mikes are called "lavaliers," although a lavalier is properly a mike suspended around your neck on a cord.) To minimize feedback with a clip-on mike, you'll need a "unidirectional" element.
Wireless means that you do not trail a cord connecting your body to the rest of the sound system, but wear a small body-pack transmitter, which sends radio signals to a receiving unit (which plugs in to AC current and to the rest of the sound system).
The world of wireless mikes is divided into two parts: true-diversity systems and all others. For a performer who plans to use a mike in various settings, a true-diversity system is essential. Anything less will be unusable in at least some environments, allowing ghastly popping noises to intrude on your performances.
Other helpful (but not essential) features to look for include:
a volume control (otherwise, you control volume at the sound system's amplifier)
a convenient way to attach the body-pack transmitter to your clothing (some require a belt or pocket, which you may not be wearing!), and
jacks for both "phone plugs" and "XLR" connectors.
For a true-diversity, wireless, clip-on microphone with a unidirectional element, expect to pay about $500 at a music store (the kind of store with lots of electric amplifiers). Ask for return privileges, in case you're not satisfied. If refused, go elsewhere!
If $500 is out of your range, you need to ask yourself whether you can reasonably be attached by a wire to the sound system. If you always sit as you tell, for example, this may not be a problem. In such cases, you can do well with a wired clip-on with a unidirectional element, which might cost as little as $150.
adapters for systems requiring a female XLR or phone plug
extra batteries for the body-pack.
To best combat feedback, you'll need to learn about equalization. The article on Sound Systems covers equalizers, amplifiers & speakers--and dealing with feedback.