You won't go far wrong with fish, blood and bone sprinked around the roots, but something with more Potash would be better to help fruiting. Although there are several fertilizers sold especially for fruit trees, the formulation is pretty much the same as potato fertilizer (which is usually much cheaper!) In general you should look for ones with high K (Potash) with N (Nitrogen) and P (Phosphorous) being lower and about the same as each other, in the NPK ratio on the box. Something with N-P-K of 5-7-12, or 5-5-10. If in doubt, ask for potato fertilizer!
Tree roots will cover an area about the same diameter as the branches above, so spread the fertilizer evenly over that area - avoid putting it all near the base of the trunk where it will do very little good.
