I envy you for getting so many years before gray hair... I had to start coloring in high school because the gray hair was already embarrassing.
I've had the opposite experience some are stating with the professional colors- I have found that the cheap ones in the stores actually last better and make my hair feel better. I worked in a salon for a few years and it was funny because even other stylists would notice it with my hair and there was just no denying that the "great" expensive professional dyes just made my hair gross. Recent years have also brought many advancements to the over the counter dyes, often they do give you subtle highlights now.
OP- when you start coloring your hair, I don't know how often you shampoo now, but try not to do it more than every 2-3 days if you can. It will make the color last better.
I would just go pick out a box you like and go for something just a touch lighter than your hair now. If you find that your hair is between two colors, if they are from the same line, generally you can mix them (take one of the developer bottles, and half of each color bottle, then save the remaining developer bottle and two half colors for next time).
If you have additional questions, there will be a hotline you can call on the instruction sheet in the box. Those people are often VERY helpful and a wealth of information.