Hi Deb, I would suggest you do a search in Google on how to plant trees.
I just planted some Crape myrtles I bought a month ago at Home Depot, and all I did was make a hole to the same depth as the pot they were in, but twice as wide. I didn't amend the soil (because there was a wasp's nest in my compost and I couldn't get to it!!) but I see crape myrtles around my neighborhood that seem to thrive with little or no care.
I plan to water it frequently from now until the soil freezes, and I also plan to mulch it when the ground freezes to help it survive its first winter.

Hope this helps!!