Make sure that you're feeling solid paddling class IV and the get with some class V friends and have them take you down and of these:
Clear Creek of the Ark - it's not that hard, but much more continuous and harder to scout than non-creekers are usually used to, make sure to have some good buddies with you.
Upper East - this one is in the CB area. It's mostly slides, but it is a good warm up for OBJ and Daisey.
Bailey - technically a low volume river, but that's nither here nor there. It's a very good intro to creeking - you can scout and walk all of the class V.
Boulder Creek - the section above and through the Elephant Buttresses rapid is a great beginner's creek. It's all roadside and not to hard, but teaches you the most important aspects of creeking - keep your bow up and always lean into rocks (not away). Running laps on this at 60 cfs is what gave me the mad skills I have today
I'd also recommend Gore, Lower Narrows on the Poudre, and Cross Mtn gorge as good beginner's class V - although they're not creeks, they will teach you safety and valuable swim lessons.
I've never run 3rd gorge on Lime or 1st gorge on Escalante, but i've heard those are easier creek runs too.
Finally, if you have the time and want to practice waterfalls, go to mexico. The San Louis Potosi and Veracruz regions are a mere 35 hr drive from denver and contain more "beginner's" waterfalls than anywhere I've ever heard of. Most importantly, the season is from now until mid-January.
Two last pieces of advice. 1) practice following guides through easier rapids without scouting them - you'll need this skill later. 2) after running some of the above, go scout OBJ at low water. At low water all of the moves are easy class V. That advice does come with the warning that many many people have broken ankles, backs, faces, and ribs on almost every drop - so "Oh Be Careful"