Hi, I just installed a shower in my basemant but at night a sewer oddor comes out the shower drain, what did I do wrong?

A fixture is connected to the plumbing system with a fitting that retains a measure of water to keep vapor and fumes and odor out. It separates the plumbing drain system from the atmosphere inside the home. Many use only an elbow thinking they want a smooth drain.
This video may help but only notice the trap, don't do the way they did, they made many errors. How to Replace a Shower P-Trap (http://design.hgtv.com/bath/Video_detail.aspx?id=917)