Right-click on your composer window and choose 16×9.
As for importing graphics and such, they will need to be formatted into 16×9 SD video size (prior to importing) in order to allow proper importing.
You’ll _not_ be able to import a square pixel wide screen image without changing its aspect ratio when working in 16×9. It’s a pretty old unfixed issue in Avid, unfortunately.
As a workaround, you can switch your project to HD, import a square pixel widescreen graphic, switch back to SD, and transcode the imported file to SD.
You might also copy and paste graphics into the Title Tool, which will correctly create 16×9 titles. Andd there’s also Pan&Zoom for stills.