try going to Goldshire and finding a mineing trainer there. Either that, or read the text if there is any given by the guy in Stormwind.
blacksmithing is a primary profession, and you can only have 2 primary prof's...your other being or soon to be mining.
Fishing, First aide and cooking are secondary proff's which you can have all of if you like, in addition to the 2 primary proff's.
....check your spell book...could it be you have another primary profession...like Herbs/skins/alc.....