I would turn to evidence-based research to answer the question, as opposed to any personal story. The FIT trial included over 6,400 women who took Fosamax for 3-5 years. Bone density at the hip increased 3.8-4.1% and bone density at the spine increased 6.2-8.3% compared to patients taking a placebo (which was calcium and vitamin D alone without Fosamax). Most importantly, patients taking Fosamax were 30-56% less likely to sustain a fracture during that period of time compared to patients taking the placebo.
So to answer your question, Fosamax does improve bone density and decrease fracture risk. Most people will take the medication for 5 years, followed by a 5 year "drug holiday" or a period of time in which the medication continues to protect your bones without you needing to take it. Because you are starting medications at a young age, discuss ways to minimize possible risk of long term side effects with your medical provider.