[Cross post from Quora]
I’d make sure you know what you are getting into. SEO is complicated and multifaceted. You need to be qualified to work for clients. I’d start with the following guidelines and afterwards, search for other free guides and tutorials and consume them all: The Beginner’s Guide to Search Engine Optimization from Moz
The next step is to gather all the tools you’ll need to be successful. Check this out: World’s Best SEO Tools and Free Search Software
As far as salary goes, it has a large range, mostly based on your experience. If you have no experience, but know enough and have the tools, I’d suggest getting some SEO projects done so that you build your experience and confidence. Check out Upwork, the world’s largest online workplace for freelance gigs.