Most inventors shipped with Blue Flashforge Buildtak applied to the build plate. The newer shipments give you the option of applying it or using something else. Just keep in mind - you need SOMETHING on the build plate to get the PLA to stick. PLA does not stick to glass alone.

Your cheap and free options are as follows:

If you'd like an upgraded build surface, you can explore names like GeckoTek, Buildtak, PRINTInZ, and the like. Note that your Blue included sheet is basically Buildtak. Please note that if you choose to use an upgraded surface that requires any sort of modification of the printer - you're on your own. 

No matter what build surface you're using, the thickness may be slightly different. If switching from one surface to another, remember to re-level your bed and watch that first layer to be sure it's the correct height!