ok where to begin? you first must have a version of maya that can render so PLE (personal learning addtion) will not do.
ok with that done open up your first blank scene.
on your menu bar at the top by 'display' expand: "window-> general Editors-> Visor..."
with 'Visor' clicked you'll be confronted with a window with folders down one side and some paint FX on the other. Scroll down till you find a folder called 'Plants' or 'Grasses' (in this tutorial I'm using 'Grasses') click on a folder to view it's contents
now with a paint FX selected go back to your scene and just draw around your graph in a clump
Now once we have a bit of grass/plant that we want to render as a decal alpha texture we need to get the camera as close to a side/profile view of out grass/plant/tree/what ever (press 'w' to get out of the paint effect and hold down alt-key and use your mouse to move around in maya)
Now we have the postion of where we want to take the image we need to set up the renderer so it also includes an alpha. to do this we need to hit out 'Render Globals' window
Ok with the render options open we just need to change the file type from what ever it originally is (IFF is default I think) to TGA
now we hit the render button this is the first icon out of the three by 'render Globals' (looks like a blank clapperboard)
With the render window open make sure it has also included the alpha channel.
With your image done and ready to be resized and brought into your game go to 'File-> save as' and save it as a TGA or TIFF load it into your art programme (photoshop/painter) and make any changes you need to make. You will most probably know the file format your game reads. So have fun making grass (and you can't get arrested for it).
Hope that helps