downgrade el capitan to mountain lion

There is no downgrade path. You can purchase Mountain Lion in the online App Store, I believe. You cannot download Mountain Lion unless you had purchased and installed it in the past. In this case check your Purchases page in the App Store.

Make sure you have good backups.

Do I need to revert my OS back to Snow Leopard first and then upgrade to Mountain Lion?

Yes, and then upgrade to 10.6.8 using the combo update below.

10.6.8 Combo Updater

Do I need to pay the $19.99 through the Apple Store to buy Mountain Lion?

s there any way to download Mountain Lion since the store only offers a disc?

If you haven't purchased Mountain Lion before you will have to order it through the link below. You will be e-mailed a redemption code so you can download it from the App Store.

Mountain Lion