Building Tools
Land Editor
In world, you can use the land editor to reshape terrain, if you have permission. The surface can be selected and then raised, lowered, smoothed, or roughened by some amount. On a private island, the owner can make really dramatic changes. On the mainland, the limits are usually much lower, such as up or down 4 meters.
If you have permission, you can also change the texture (images) for the surface, such as sand near the water, grass on the plains, or rocky mountain tops.
Plants (Flora) are not part of the land. They are objects that you place on the ground. There are plants in the Inventory Library, and there are many more for free or for sale. Make them using the Object Editor.
Bull Dozer
The Bull Dozer is a special mode of the Land Editor. Instead of selecting land, you run a small bulldozer over it.
Land Editing scripts
The scripting language has commands that will raise or lower the land. You can create a sort of custom bull dozer that goes around your land and sets the terrain heights to whatever value you want.
External Land Editing Tools
On private islands, the terrain map can be downloaded by the island managers. It is a kind of 2D graphics file, a 13 channel RAW file. Details can be found elsewhere.
You can edit the file in various tools, and upload the result to do a complete remake of your islands.
2D Graphics
There is no 2D graphics editor inside Second Life. Instead, you use any one of the many editors available outside, such as the GIMP, Photoshop, or even MS Paint.
3D Modeling
See this 3D modeling guide for detailed information.