I want to create a landscape in blender for using in a game.
I don know how can I do this ?
I mean my export file what can be?

for making 3D game in bender anyone can help me?
I'm very interested but very confused?

Games want data in a specific format, so the answer depends on what game.

In a nutshell, you create your content in Blender and run an export script to export the data in a format your game can use. You can enable various exporters in User Prefs->Add-Ons.