I need advice on roadbikes or biking/cycling in general?

I have always wanted to bike or cycle more seriously and my friend got a roadbike from a garage sale and wanted me to bike w/ him. I looked on Craigslist for a used road bike and found one for 250, it is a Centurion Diamondback

What I’d like to know is how can I learn more about biking, repairs, maintenance, materials, brand and types. I want to know about what different frames or rims, gears, wheels do and their quality.

Is there a good biking/cycling community/forum or website?

Thanks =D