Looks like a 20 gallon high, dimensions 24" long x 12" wide x 16" high. To make a wall, you get plastic craft mesh or other safe material and sandwich moss in between two pieces. The moss grows out and over the mesh, eventually disguising it. There's a couple threads on here, with pictures and such. Off the top of my head I think msjinkz has a moss wall.
http://www.aquarticles.com/articles/plants/leong_Moss_Wall.html
Not to assume so much, but I'm guessing you just have stock lighting, which is probably only 15 watts or so. In such a set up it will take a long time for your wall to fill in, and you will have to be diligent about trimming the wall. If it gets too thick, the moss will grow in such a way that it might prevent the bottom and inside of the wall from receiving light and the moss will die >.< So in other words, it will definitely not be maintenance free. I have heard of people "angling" their wall out from the aquarium glass, so that the bottom of the wall can get more light.
In the lower light, java moss tends to grow in a "stringy" way instead of a branched "bushy" way. I have christmas moss, which would probably look better in the end. I have a 20 gal with stock lighting, I'll put some christmas moss in there and see how it does.