If you want it to be a "tree form" crepe myrtle, then trim off all the lower branches so that you have a visible trunk (like a tree). Right now it is growing as a shrub form.

The horror stories have to do with chopping off the tops, not what you need to do here.

Don't trim them. You'll be sacrificing flowers if you do. I'd wait to trim until either a) after flowering or b) in Spring right as growth begins.

Since it might not be clear to you, what Dave is saying is that CM's flower on the ends of the growth from this year - one of the reasons people get away with, and perpetuate the "crape murder' which is what you get told NOT TO DO by members of GW - which is why he suggests waiting until after flowering or doing your trimming in spring so as not to lose this summer's flowers.

You might by now be able to see flower buds forming at the branch tips and be able to decide if you want to remove the most wayward branches even so, losing the flowers. If the branches aren't too much in the way, my inclination would be to wait 'til they flower and then deal with them. IF they ARE in the way, shorten them back until they aren't, or, if you want a tree-form, take them off at the trunk.

As Esh Ga says, you can grow either a tree-form or a bush-form CM. Some of the newer, dwarf cultivars can, and should, be grown as bushes and look very nice. If you know, and tell us what cultivar you have, you can get more directed advice as to whether to trim up the trunk, or leave it as a bush. In any case, you can grow most cultivars as bushes and/or in pots for several years, so there is no urgency, other than your expectations of what you want this CM to look like. If it isn't one of the dwarf cultivars, then eventually it may outgrow the pot and the space on your deck, and need to be planted in the ground. For now, though, it looks to be quite happy and healthy where it is.

Let it flower and lose its leaves in the fall. Then prune off the lower branches if you want it more like a tree. But don't bother them too much...don't cut off the seed pods either.

Thank you all for the advice. It's a Seminole and I purchased it from Carroll Gardens online. I think it was described as reaching a height of 5-10' which I thought I would be able to keep on the deck. I would like it to be a tree form and grow into as natural a shape as possible while providing some privacy. I think I understand about when to prune, it's just which branches and how far back I'm unsure of to encourage the tree form.

(Now onto my next posting, I don't know if you can see the cherry tree in the second pic, but it doesn't look good. I'll be posting pics of that next).