How do you do slide show on apple tv

There are a few options.

If you wish to use your full library you will need to do so through home sharing.

Setting up Home Sharing for Apple TV (2nd generation and 3rd generation)

You can also setup a screensaver via Apple TV settings. You can choose to use your photo stream or select photos through home sharing (approx 100 images will be temporarily cached to the device)

If you want to create a custom slideshow on your computer, you can create a movie and home share via iTunes. Mirroring is another way, but you would need to use a 3rd party program (i.e. Airparrot) since your computer doesn't meet the requirements for that.