Colorado Whitewater Assoc. is a volunteer group that runs a number of instructional classes (mostly done by 31 May) and whitewater weekends:
Colorado Whitewater - Home
The Poudre River weekend is happening June 26, 27 is something you should check out - guided trips on moving water - you probably want to do Filter Plant as a start.
Meanwhile, if you want a warmish place to practice your roll, Bear Creek Lake park in Lakewood isn't a bad choice - roughly at C470 and Morrison Rd.
There is an entry fee.