The most well known qualification in the industry is CELTA. There are other qualifications, but please be careful to check that they are being run by reputable organisations.
Here are a few useful Websites to get you started:
Dave's ESL Cafe
One of the biggest sites for EFL teachers and plenty of jobs.
Probably the biggest EFL teachers job Website on the net.
Lots of hints and tips on finding jobs in this British newspaper.