The numbers on a fertilizer bag tell you the percentage of N-P-K.
10-10-10 means 10% of each element, so for a 50 lb bag you are getting 5 lb of actual Nitrogen, P, and K.
If you take this info to your co-op and tell them what you need, they will point you in the right direction. It doesn't have to exactly right. As a novice, you should make friends with your local dealers (seed, fertilizer, chemical, etc). They are usually great people and very helpful.