Yes I'm new and im obliged to ask this

Maybe look in our Tutorial section or google it.

You can't just place entire structures and even if you could there's no garauntee that if you downloaded some prefabricated structures in order to do just that, that the order you place them in/arrange them in would be balanced and would thus still require a level of personal customisation. Even just to join them up and seal the world.

The beauty of Hammer is the capacity to create truely unique environments, this isn't a console based world builder like in Time Splitters where you only need to throw down "tiled" sections of map.

Open a new file, click and drag in your 2d views, right click and select "create brush". This is your first object, world geometry, otherwise called a "brush". mousing over the camera (and hitting "z") will allow you to control the camera view like you would in any FPS game, you will need this to navigate and see what you have created in 3d, much like a preview. Often if you've built your initial brush at the origin the camera is inside it and this can be confusing at first as you cannot see what changes you have made. Back the camera up a bit so you can see the green red and blue axis markers.

http://developer.valvesoftware.com/wiki/Your_First_Map
Work your way up from there.

Yeah, Hammer is a real development tool, the very exact same one Valve used to make the game in the first place. You can be sure that maps like Dustbowl weren't built in an environment that's anything like Halo's Forge.