OCTGN is a great free option for people looking for online games of UFS. Although, it does take a bit to get up and going. We're here to help.
Here is the OCTGN site itself. That's where you start. Then you're going to want to check out the basic tutorials so you have a little understanding. They have a video tutorial on their home page.
Next you have to get the image packs together, and install them. Here is the link.
After they're installed, you just have to put a deck together and find someone to play with. Currently OCTGN is up to date with the Mortal Kombat set.