I personally got a TESOL certificate before coming to China and it was a requirement for the job that I initially got. If you do decide to get a certificate I recommend taking the cheapest quickest course possible. From what I've heard people who have taken 20 Email Database hour TESOL courses get the exact same preference as those who have taken 120 hour course. I think the average price for a TESOL course is a little over $1000. The following certificates are all acronyms that are seen as basically the same from employers (TESOL, TEFL, ESL).
Degree - This is one thing I don't have, (I instead have a 2 year diploma in video game design), and it has made things very difficult for me here in China. The Email Database big attraction for coming to China was that you didn't need a degree to get a working visa, a TESOL Email Database certificate was more than enough. After arriving here however I was told there was a new policy and now 4 year degrees are required in most locations. If you have a 4 year degree then you don't need to worry, but if not make sure to read the section of Visas below for more info.
Experience - And lastly they look at your experience. Obviously the more the better. Don't be discouraged however if jobs boast they require teaching experience apply for them anyway, you'll often be considered despite not meeting the requirements. If you have your Email Database heart set on English teaching and you don't fit into the ideal of the above categories than please don't let it discourage you.