Joe and Jerry make good points on their class recommendations. A lot depends on your budget and goals, but the worst thing you can do IMO is race against no one.
Here are some sites to check out in the order they were offeredhttp://www.specracer.comhttp://www.specmiata.comhttp://www.formulavee.org
I have participated in both FV and SRF and both are challenging. Jerry races Miatas and there are lots of them. You can also rent each of these cars from various sources which can also be a good first step before deciding.