I have a feeling I should know this by now but I just don't. If it has been discussed here before give me a link, otherwise I need someone to talk to me like I'm five and tell me how I take one folder (or file?) and put another in it's place. If it helps, I've got OS X version 10.5.8
Same in any Mac OS...
Open two folders: the one where the file is that you want to move or copy (the "source" folder) and the one where you want to move it or copy it to (the "target" folder). You can actually open the target folder, or just open the folder that contains the target folder. You must open the source folder so you can see the file you want to move or copy.
Highlight the file (in the source folder, of course) and then drag it out of that window to the target folder. If you've opened the target folder, just release the mouse button (called a "drop"). If you've opened the folder that the target folder is in (known as an "enclosing" folder) then hover over the target folder and drop it. Then you can open the target folder to make sure it's there.
To control whether you move or copy:
If dragging from a source to a target that are both on the same drive, the default action is a move. The item will not remain in the source folder. If you want it to, hold down the Option key as you drag. A green dot with a plus sign in it will appear to indicate the item will be copied.
If dragging from a source on one drive to a target on another drive (external hard drive, or a flash drive, etc.) the default action is a copy. A green dot with a plus sign in it will appear to indicate the item will be copied. The item will remain in the source folder. If you don't want it to, hold down the Option key as you drag.