For many older systems, there are very few resources available. Lubuntu is the best distro for many of these systems, due to the low RAM requirements.
Go to the Lubuntu website and find Download. Click on Desktop.
For all intents and purposes, most of these systems only support 32-bit distros. For systems with <3GB of RAM, download the 32-bit version. For systems with >3GB or more, download 64-bit (if your processor supports this).