A picture would be great, but what you are describing sounds like a bad case of black spot, a fungus disease. Fungus problems need to be prevented, rarely cured. Many varieties are very susceptible to the rose diseases. Remove all damaged leaves and destroy, including any on the ground. Use an approved fungicide to spray the remaining leaves. Any further signs of the disease should also be removed and destroyed. This can defoliate a rose bush which has no resistance. An established rose can survive a couple of seasons of this, but looks bad. If you plant on getting another rose, consider one of the great very disease resistant fragrant ones. So much more fun to grow.