I installed a sink and shower right next to each other. The flooring is not yet in. The sink goes in one side of a T the shower the other side and the third side of the T goes to the main Drain. When I drain the sink it backs up into the shower. How do I stop this. There is a trap under the shower - I thought that would prevent this - I guess I was wrong - luckily I used plastic pipe. Anyone know where I can get a diagram of the proper setup.

I installed a sink and shower right next to each other. The flooring is not yet in. The sink goes in one side of a T the shower the other side and the third side of the T goes to the main Drain. When I drain the sink it backs up into the shower. How do I stop this. There is a trap under the shower - I thought that would prevent this - I guess I was wrong - luckily I used plastic pipe. Anyone know where I can get a diagram of the proper setup.

Hey Art,
What you have done is drain your sink which is higher directly into the shower which is lower. What did you think would happen? A trap just protects the fixture from escaping sewer gas. It is not a check valve or a backflow preventer.
If you can scan and send me a drawing of your fixture layout and the sewer main that you're connecting to I'll lay it out and fill in the dots for you.
Regards, Tom