**The first thing to do is register your work under the copyright of the Library of Congress.**
**After that be sure a visible watermark is across each image that identifies you clearly.**
Now that this is done you have choices.
A blog is an easy and free way to show your art. I sell a lot right off my blog. But it took a while of constantly posting to get a following. Just keep it up. It will be your online portfolio. Do not remove sold work just mark it sold.
As for public forums- are you looking to hang out with fellow artists or to sell?
For a good community I recommend WetCanvas.com
For sales choose someplace that does not charge if possible. Tell me who you are looking at and I will expand. I have tried dozens of sites.
If you are looking to sell prints I highly recommend ImageKind.com
Let us know more about what you want to to and who you are looking at and I will be glad to give more suggestions.