A good way to know if a river is the right difficulty level for you is to use the keelhaulers website. You can self evaluate your skill level and match it to a set of rivers usually in a progression of difficulty which is explained on their site. It may not be a perfect system but at least you can have a reference point. Things to consider include your skills and the group you will be paddling with. Just get a number and see how much you want to push your self. Pick a set of rivers start looking at American Whitewater.
Here is the link for the self assessment:
Once you have that number take a look at this chart to see where you stand.