Hiw can I assign a follower as my steward

You just ask them to be steward.
Does anyone know if its possible to change steward if you've already asked someone?

The follower has to be in the same cell as your house though, and you have to have the second area of your house built.

I'm curious about which races can be stewards though.

I can confirm Nord and Redguard, I haven't tried the others yet.

I would personally want Serana as mine since you can't marry. xD If anyone has any experience with the rift piece of land post on my recent topic would be appreciated.

I just got Ghorbash to be my steward, and I think everyone on http://elderscrolls.wikia.com/wiki/Blades_%28Skyrim%29 can become steward. But I haven't tried them all out yet.