Behance is the best online portfolio site, besides a personal website, but it is for people with an account with adobe. Most everyone I know has photoshop, or the suite, so I suggest you get a account because it's an amazing networking tool and it's a very professional place to send clients.
Tips:
1) Keep track of your process. Take photos of your process or save your file as jpegs as you go along. One of the biggest ways to draw people to your profile is showing how you went from A to B.
2) When you order the images, put your final image at the beginning and end. Seeing what the final is at the beginning lets people know what they're in for, and then you show the process shots, and then the final again just to complete the thought.
3) Add description. Make sure to write even a little on how you achieved the piece, the more detailed and lesson like the more people can get from your work. People want to know how something was done not just bask in the aesthetics.
4) Tag your work. Label what tools you used and the fields the art is part of when the site asks you for. This makes it easier for people to find you.
5) Share. When you're done publishing you're prompted to share to different site, share to all of them. If you don't have a art account on every social media platform, go get one now. Then link them all together. Then post like your life depends on it.