1. Right click on the plane. go convert to -> and select convert to editable mesh.

2. on the right hand side click on the red square

3. now select the move tool

4. on the z axis move it up/down.

moving it down u can get a trench, moving it up u get a hill.

look at eqvalisers tutorial, you obviously havent read it.

ps. if you want random hills/trenches use the noise function (search back like 4-10 pages on the forum for my topic on how to use it.

ps2. you can also make a trench, with extrude, also search the forum for my post on how to extrude.

hope this helps you