Start by clearing your artboard. Next click window ==> artboards. A new panel will pop up on the far right of the screen. Click the menu button on that panel (3 horizontal lines stacked), then click new artboard.

Now select the artboard shift tool. Adobe will automatically select the most recently used artboard (in this case the new one). But you can select any artboard you want. The move tool allows you to move them around the workspace, change their size and shape and align them as you see fit.

This tool is the easiest way to manipulate artboards on a working document.