We have Delta brand tub/shower fixtures, the one handle kind. The problem is that water comes out of both the tub faucet and the shower head, even though the shower is not turned on. This happens every time the tub faucet is turned on. But when you turn the shower on, all the water comes out the shower head like normal. What can I do?

Are your walls still open? I assume they are as you'd be unlikely to close them up without having had tested your water flow. Here is what I would do.
1.) use the orange plastic C shaped release tool to depress the white plastic ring on the shark bite fitting that you have on the delta side of the Delta-ShowerHead connection.
2.) Carefully and slowly turn water on and see if water is exiting the fixture where you expect it to.

IF yes, water is coming out of the delta fixture then you know the flow problem is somewhere between the delta fixture and your shower head.
IF no, water is not coming out the delta fixture then you need to re-access how you connected the supply lines to the delta fixture.

Not usually, but there should be an inverter or other unit to activate the shower. Often it is in the tub outlet and blocks the tub outlet to force water up the shower pipes. If there is no such or if it is just not working you may need to run the piping up to a sufficient height and then back down to the shower head to create enough pressure resistance if it is as you say, requiring a specific height of pipe to get a specific back pressure. But there also needs to be a mechanism to force the water up the shower pipe. Look up Delta on the web and get a phone number as there will be someone at Delta that can fully advise you about this.

Take the handle off and adjust the hot temperature stop. Its just a plastic piece that needs to be adjusted up more. It just fits in grooves like a gear. Its easy to do.

easy way is to install a shutoff valve on the shower head.

Sounds like the delta people forgot to install the diverter piece inside of valve body,small white plastic piece screwed into valve. If you take out valve you should see it up inside body

Is there a button on the tub filler? How about the spout on the tub, does it lift up? If there is not a tub spout then it should be shower only, if it has a tub spout it has to have a lifter of some sort, the foto I found showed the little button lifter. Hope this helps.

Two possibilities come to mind. 1. The shut off valve (in the wall under the fixture) is almost closed. 2. Trash in the valve assembly or a broken seal. Turn off the water at the shut off valve and remove the handle and stem from the fixture. Clean and replace anything broken. You could even open the shut off valve for a second to flush any junk out of the fixture. Reassemble, turn water back on, and test. This should solve your problem.

check the tub faucet the pull lever maybe slightly faulty causing back pressure in line and pushing water up to shower head --u may have debris in tub line causing partial blockage -or u have alot of water pressure coming in and it is to much for the lines to handle and it is pushing up to shower head

The only possible explaination is that the water is finding it easier to go up than down. Water is lazy, just like me. ;)

Use an allen key to remove the tub spout. It's underneath and awkward. Once the allen key is loose, the spout just slides off. Try running the water. This may solve your problem and all you'll need is a new tub spout. If not.....

Turn the water off to the house.

Use an allen key to remove the handle.

Use a phillips to remove the trim plate (2 screws). Cut the silicone around the trim if it's there (which, in a proper installation it shouldn't be, but...)

Unscrew the nut (usually chrome) holding the cartridge into the brass body of the valve. The cartridge pulls straight out. Check for restriction in the ports of the cartridge itself. Check for restriction in the line going down to the tub spout. If you can't find anything, replace the cartridge.

Reassemble & Enjoy!

well the Diverter is not working it dose not matter the distance. when applied it should shoot the water to shower or tub if not working will come both.