The hairdresser will usually tell you :) if your unsure about anything, just ask!! They'll always be happy to give you an answer. Also, getting your hair dyed at a hairdressing salon is wayyy better for the health of your hair as the chemicals are less harsh than if you dye your hair at home
Yes, usually a salon will give you some great highlights, so that it looks a bit more natural than you can get at home. Your hairdresser can show you a chart and make some suggestions if you aren't yet sure when you get there.