It takes some time to get green or black. Black will take atleast 3 years of project work. Green will take some time too.
If you put on your resume, "Six Sigma Green Belt", you'll likely be asked about the various projects, you worked on. The six sigma tools used and outcomes associated. They will want you to be specific.
There are formal classes you can goto and get a green or black belt. (A coworker did a black belt class, cost him 10,000).but the problem is, without any project expierience those green or black belts are more like white belts and are frowned upon.
Just be careful with your approach to this. You goal should not be six sigma just to get a job. You goal should be to fine a job, then let you work on six sigma projects, get trained.. so when it is time to move on you're much more valuable.
Six sigma folks treat it like a fraternity.