I want to make a map that is tiled, and I want to start the map with all grass.
I use this tile:
When I display this tile and place another next to it I get the following result:
How do I get it to be "flat?"
It looks like you're not properly calculating the offset in the height from one tile to the next. The offset needs to be set so that two tiles next to each other form a rectangle, like this:
Just adjust the height offset until there is no stair-stepping like you can see along the edges in your image. For this particular tile it's 25 pixels.
It looks like the edges of the green tile are shadowed so that it looks more like a box than a flat diamond. If you can edit the tile just make the entire diamond the same color or at the very minimum make the shaded area the same color as the top.