gas milage

Just bought used 2011 lacrosse CXS and it appears to be consuming a lot of gas per mile than it should!. What should I do?

So I purchased my first car in July of 16 and it was a 2000 Buick lesabre custom. Traded in my 89 Park Avenue as it was nearing its end of life. It had some problems with power locks and the right rear window but all could be fixed, and after all that I love this car! My question now is gas...