It is best to wait until fall, that way you will not be having insects target in on the open wounds and vectoring in diseases.
You do not have to prune it heavily each year, but pruning does help encourage new growth, and will help thicken it up. When making any cuts, be sure to prune all the way back to the branch collar. This is the round swollen area that kinda resembles a toilet seat that the branch comes out of. Prune as close to even to the face of the collar as possible.

1st thing is to remove any sucker growth (the branches which appear to grow straight up from the trunk or branches.
2nd you can remove any lower branches which make mowing a hindrance, then the rest is pretty much up to you and how you want to shape the tree.
Just remember you can always go back and cut again, but once cut, glue does not work too well to fix any mistakes

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Mike Maier
aka
The Plant Doc