There are alot of different paths to take.
Most people use the sheet-like backgrounds for their tanks. If done right, a "scenery" like background can help tie the tank together, I don't care for these much though. Another option is a solid color background (usually blue or black) which help bring out colors in both fish and plants.
And a third option is building a 3-d background for the tank. Of course this takes more work, but it can look great! I'd suggest googling rate my fishtank and take a look at the different kinds of background people use.