Agree with Byron. Read his guide first, and then you can look into the soil substrate guide in my signature if you're considering setting up a new tank...
Once you get the supplies (flourish, proper lights, etc) or set up a new tank (soil substrate), many plants work well.
My absolute favorite at the moment is Monosolenium Tenerum.
It does tend to grow into a rounded shape, but once a month or so I pick it up off the substrate and shake it-
It falls apart, settles over the aquarium, and sinks- A day later it changes bright green and looks like a nice combination of grass/moss. Can also be sold on the classifieds pretty easily.
It naturally sinks, so there's no need to tie it down, and you don't have to worry about fish moving it around... Seems to enjoy abuse.
Other good mosses and grassy plants to start out with are dwarf sag (planted in the substrate), dwarf hairgrass (planted, does best in sand or extremely small-grained gravel) and Java moss (Have to tie it to rocks or wood, or can place a few pieces of gravel on top to hold it until it anchors itself.)