If you want to run a modern system with an old computer, you have to compensate some of the missing GHz and megabytes with your own brains. You have to know what you are doing. You have to know what you want your computer to do. Only so you can build a system that is ideal for your needs and your hardware.
This means you have to be ready to learn something about Linux and how it works. It is not rocket science, it is something everyone can do. If you are willing to learn, you can realize the full power of Linux.
We are lucky to have a lot of free documentation. There are even many good introductions to Linux. Most of them are published by The Linux Documentation Project. I suggest you first familiarize yourself with the site just to see what kind of documentation there is. Depending on what you already know, select some of the guides where you can learn at least something new. If you are relatively new to Linux, you might start with Gareth Andersons GNU/Linux Command-Line Tools Summary or Machtelt Garrels Introduction to Linux - A Hands on Guide.
Just remember: learning is fun and knowledge is power!