We want to do some cool canyons but have absolutely no experience.
Let me fix that for you:
We want to do some searingly hot canyons but have absolutely no experience.
Anyway,
I wouldn't recommend Canyonlands in July. Trust me, it will be miserable (and not to mention dangerous). The wet canyons of Escalante and Zion National Park would be much better. The closest canyon to Canyonlands that I would recommend in summer are as follows:
Professor Creek, Mill Creek and maybe Negro Bill near Moab (waterfalls and swimming holes). Couthouse Wash (wading pools) might not be so bad either.
Dark Canyon between Black Steer and Lake Powell won't be so bad (lots of waterfalls/pools), except for the hike back out. It's rugged country though, so it may not be the best for you. Same with the waterfall and pool laced Gypsum Canyon.
Other than the above, I'd really avoid the Canyonlands area in July (unless you want to head up into the La Sal Mountains).
In Escalante and Zion National Park, you would have much more options. Some good summer choices area below.
Escalante:
Death Hollow (coming in from the top is harder, so go in from the bottom if inexperienced) - nice slides and pools.
40-Mile/Willow Loop - wonderful pools and falls
Boulder Creek - nice swimming holes
The above are best for summer, but any of the well watered canyons would still be possible (i.e. Coyote Gulch, The Gulch, etc.).
Zion:
Zion Narrows-always great in early July, nice later on when flash flood danger is lot. Best in hot weather; the hotter the better.
Subway-from the bottom is easier than from the top
Oderville is a good one, but it does have some scrambling and one small drop that is best with a rope. You can hike it from the bottom.