Q: el cpitan won't download from app store

First, exactly which year/model iMac do you have?

And, have you used the app store/purchased or downloaded things with your Apple ID previously?

Okay, since you've never used the app store, make sure you are using the Mac App Store (not the app store for iPhones, iPads, etc). Since you've never used it, have you created an Apple ID? If not:

https://support.apple.com/en-us/HT203993

and:

https://support.apple.com/en-us/HT204316

After that, restart your Mac. Then open the App store app (in your dock and/or Applications folder) and you should be able to get to El Capitan.

FWIW, you are running Mountain Lion (10.8) right now and getting to El Capitan is considered an upgrade as it is a totally different OS version. Please remember to make a complete backup before attempting to upgrade.