This usually depends where. If you have the right qualifications then you can pick up work in your home country quite easily. Schools will often employ several local NNS (non-native speakers or those whose Mother Tongue isn't English) for each NS (native speaker) they employ.
Getting work outside your home country is a little more difficult. If your English is very good and you can class yourself as bilingual (maybe even if you're not quite) then you can often find work - especially if your other language is something the students might also be interested in learning.
But above all - regardless of your qualifications - you will have to demonstrate to your boss that you can speak English to a very high degree of fluency.
In an ideal world, you will need the following qualifications:
It will also help if you have experience!
Regardless of how good your English is and regardless of how good your teaching ability is, one major problem is that school owners will immediately assume that a NS will make a better teacher than a NNS. Thus if you apply for a job online you may well find that the school will see on your CV/Resume that you are not from an English speaking country and immediately disregard your application.
Unfortunately there's not a lot which can be done about this when applying online. However, there are few strategies you can try to get your foot in the door of a school. Aside from the usual methods of finding work try these ideas:
Remember, however, that some countries have rules in place for employing native speakers. No matter how good your English you cannot get a job in South Korea unless you come from one of the major English speaking countries and are a native speaker.