MacWorld has good coverage on the step you'll need to take:
"If you’re running OS X Leopard (version 10.5), you need to upgrade to Snow Leopard before you can upgrade to El Capitan. Unfortunately, you can’t do it through an Internet connection and it will cost you $20. You have to buy the Mac OS X Snow Leopard installation disc from Apple. After you upgrade, you can then upgrade to El Capitan by using the App Store online."