I ranked my own sites first for some competitive keyword terms then offered
SEO to prospective clients at a very low fee.
However
SEO is good for some businesses but a waste for many others.
That may be the biggest hurdle you will face....knowing when to say to a client, I like you and your money, but you won't like the end result. Turning away those clients is hard to do but at times very important.
#1 rankings are very nice, but when they don't generate leads or drive sales, then it's akin to giving a starving dog a rubber bone.
As for certification, until an accredited college / university has a major course selection in Search Marketing, what most have is a piece of paper from someone, who thought it better to try and teach
SEO, than actually practice it.....
Good luck in your quest..