The first thing you need to know is that it is expensive and time consuming to become a Microsoft Certified Trainer and you need to renew your certification on an annual basis.
The full requirements to become a certified trainer can be found on the Microsoft Learning site here. Don't forget you also need a year's experience on top of all that. If you have your MCP status, you can apply for MCT status on the mcp.microsoft.com site.
The plus side of being certified in this is that you can work just about anywhere in the world, good trainers are in high demand.
Being a trainer comes with a level of responsibility though. You owe it to your delegates to know your product and give them the best advice possible. Remember that whatever you teach them on SharePoint they are going to implement in their organisations. Giving them the wrong advice makes you responsible for them making serious mistakes and messing up their entire site collection. I have seen this happen myself at very big clients.
If you are a business looking for trainers, make sure you do your homework about the level of training you are about to receive. There is a lot of ignorance in the industry in this field and some people may be inclined to take advantage of that. The courses are expensive, make sure you are getting your money's worth.