Help whit a ENGINE 3d Terrain Selection

Plz im tink a so long but no idea how to edit a terrein, using my Engine whit a mouse click or pressing a mouse, any one have ideia whow could i make this?

Realy this is very important.
Tnks.

How is your terrain being represented? ie heightmap, 3d model, random perlin noise generated, etc?

-SirKnight

Let me tell my idiea …
My idea is build a terrain form a IMG and try to building a WORLD for a MMORPG, but i need to make a tool to edit a world in real time, i have a ground and the ground have a size not definide…
To make the terrain up down … cliking in the vertex of poligon…