all the saves are saved in the minecraft folder in the "saves" directory. If you don't know how to get to the minecraft folder:

On windows, just copy and paste this into the Search or Run box:

%appdata%\.minecraft

And hit the Enter key.

On Mac OSX, the easy way to get there is to just paste this into the Spotlight search window and hit the enter key.

~/Library/Application Support/minecraft

On Linux, you can get there by using the ~ shortcut which represents your user folder directory.

~/.minecraft

once you find your save directory, you can copy the contents to the same place on your new computer. Hope this helps!