First, your iMac will run El Capitan (10.11), provided that you have at least 2 GB of RAM installed and 8.8GB of spare disk space. If you want it to work well, you need 4GB.
The upgrade process is easy enough – purchase 10.6 Snow Leopard, upgrade all the way to 10.6.8, open the Mac “App Store”, click on upgrades, and then choose the free upgrade to 10.11.
