Yes I think I and many others here have asked the same question before taking the plunge.
In the end, it comes down to experience. I am afraid you will need to jump in the same boat as the rest of us: start roasting just to get a feel for it. Use the same bean (aim for around CS10 initially) until you think you have it right. Then start trying out the various single origin beans to see what you like about them. Trust me, its the best and most fun way for you to learn.
There are guidelines (but no rules): lighter roasts for manual brewing methods, darker roasts for espresso. There are lots of other variables such as resting period, brewing equipment, technique, black or white coffee, etc. Too much info to squeeze into a single post and there has already been lots posted on CS and other sites. Browse through various topics/areas on CS such as BeanBay and KnockBox (to see Andys notes on the beans he sells), Cupping Room, Whats in my grinder and cup today?, Blending Room etc.