To put a a/c window unit in a sliding window, you will probably have your window completely open to fit the unit. You will then need to cut some plywood or chipboard to fit the top part of the window that would be left open above the a/c (you will need to remove the screen). If you can paint the plywood to match your siding, it will look better from the outside of the house. This approach will cut down about half the light that you normally get from that window - but window units are really best designed to go in double-hung windows, not sliders. Once cooler weather comes, you can remove the a/c unit and the board and you have your window back.
Another option, although it requires a lot more work and should only be done if you know at least a little bit of house construction, is to install the window unit into a new cutout in your wall - this leaves the window still fully functional.