Pruning will induce branching and shape your tree, and make the trunk thicker. It looks like it may have been reaching for light. You should have asked for a discount when you bought "since I'll have to cut it down by half"
I would start by cutting it down by 1/2, about where the yellow tag is. Maybe do this just before avocado normally flushes new growth in your area. Don't be scared, if cared for properly it will be much happier "under new management".