Hi: I do a lot of photography and I have wondered about this as well when I was new. I would not recommend that you plaster a large logo across your image. This is quite distracting and turns potential buyers off. A small tasteful logo or signature in the bottom of the shot is often adequate. There is a program out there called Tin Eye and it can find your photograph on the net. It doesn't always work. If you have Adobe Photoshop or Lightroom you can set the metadata with your name and address in it and the date of publication. If there ever is any question over ownership of the photo it is embedded in your photo. To further prove the date of publication you could upload your photo to a sharing service like Flickr.
In talking with many photography friends the consensus seems to be to let some of your shots be used anyway. As long as there is credit given it can only help to promote you. I have had dozens of photographs published online without specific permission because I granted permission via Creative Commons. Additionally I only publish low quality on the net and if someone really wants the photo they can contact me. A friend does this for his stage and concert photography. If performers want copies they can download the 480X640 from Flickr which will give them a serviceable 4X6 print but no more. If they want 8X10s they are available for sale.
With the billions of photographs that exist on the net the likelihood that you have something extra special(worth big bucks) diminishes. Stock agencies shoot you $0.25 or so for a shot as an example. I think you are better developing a following for your work (assuming something like landscapes) and keeping those people in an email list for notification when you have new product.
I also place some of my work on Fine Art America but as for sales it is no good. I don't know of anyone making $$$ there. You can refer people here to look at your work and they protect it with software so the shot is not very usable if downloaded.
Hope that answers your question - in a long winded sort of way!