Have a look under Purchases in the App Store and see if it is downloading.
Today 20th September is probably the last day you will be able to download El Capitan from the App Store
as the new macOS Sierra is due to be released today, and in all likely hood El Capitan will be withdrawn.
You may still be able to get it from an Apple Retail Store.
Another thing to check is to see if there is a partial download of Install Mac OS X El Capitan in your Applications folder
if there is and it is less than 6.21Gbs then move it to the Trash and delete, also check in Launchpad to see if there is an icon for OS X, if there is click and hold till it wiggles, then click the 'x' to delete it. Restart your Mac, relaunch the App Store, see if you can download now.
If you are successful in getting the full installer downloaded, and before you start the installation go back to your
Applications folder and make a copy of the Install app on an external drive or USB stick. It is handy to keep a copy of this so you don't need to re-download it should things go wrong, and you can also use it to install on other macs without resorting to the App Store.
Please make a backup of your mac before doing any of this.
Next as you are upgrading from Mac OS X Snow Leopard 10.6.8, check if there are updates for that first, there was an
update for the App Store in 10.6.8. There are a lot of differences in El Capitan to 10.6.8 not just visually, but you will need to see if third party apps you use have updates available for El Capitan. You may find that if you use Aperture or iPhoto, they will no longer work. iPhoto has been replaced by Photos, and Aperture discontinued, although there are ways to get them back working.
