Before you can jump to El Capitan on a machine running Leopard, you must upgrade to Snow Leopard or Lion. Snow Leopard is no longer available from Apple but Lion still is. You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax. It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation. You will need, minimally:
1. 2GB of memory
2. 7GB of available space