This question is much more complicated than it seems. We really don't have enough information to answer you appropriately.
This is sort of a unique area of expertise of mine, having a natural curiosity and a love for experimenting, I've used over 60 linux distributions in the last 15 years, and I can promise you that there are LOTS of good server distributions.
What you need to ask yourself is what not what is a good server, but what is a good server for you?
I did the same thing as you back in 2009, I took an old 686 and turned it into a server. I doubt it would have performed well under high strain but it was awesome for my own personal use.
As a beginner, I would guess that the primary feature you're looking for is user friendliness. That's always been ubuntu's strong point.
Do you intend to run your server headless (no monitor)? Will you connect through SSH and manage through terminal or will you install a web based administration package?
Will you install a desktop alongside your server? This isn't good as a production platform but it won't hurt you just to try things out.
There's a website out there called
http://distrowatch.com which is a great resource in choosing server software.
When I was first playing around on the server I found it really easy to use some of these:
SMEServer - Robust and user friendly (for server software) based on RedHat/Centos
Puppy Server Edition - you might have to google a bit to find this one
Damn Small Linux - Discontinued development but works as a server right out of the box and has a desktop too.
Puppy and DSL are also super duper fast and perfect for older machines. SMEServer is a bit heavier software but should work for you if its just for testing. If you find out it runs too slow you can always try using a previous version. You can drastically extend the life of your computer by downgrading your software rather than upgrading your hardware.