You'll need to upgrade your system to at least 2 GB of RAM (best to go all out to the max 6 GB of RAM).
As far as getting the OS you'll need to find a friend with a Mac that had at one point downloaded it. Apple has removed from the App store availability of the older OS to push people to the newer OS's. While once you've installed it on your system from the Apps Store you can access to it again, its hard to get it afresh like what you're doing here!
OK, you found a friend who at one point had El Capitan. Have him go the the 'Purchased' tab on his App Store account. He should then be able to download it again for you (he may still have it in his Application folder). Once it downloads it will ask you to run it, you want to quit the installer. You'll find the Installer in the Apps folder, copy it and paste it onto the desktop.
Now you'll need to prep up a 16 GB USB thumb drive (GUID) following these steps How to make a bootable OS X 10.11 El Capitan installer drive.
Once done you'll use it to first test your systems drive using Disk Utility, then run the installer. Done!
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