As others have noted, earning is part of the equation. Learning to spend miles wisely and efficiently is the second part. Learning about the various airline programs and alliances will give you the necessary background. To take an extreme example, I could easily blow 50K RT on a coach seat to get from ORF to JFK--or I could spend 9K for the same seat.

Research will also reveal how to use the miles for exactly the type of travel you want to do. Biz class on international flights is a fine goal, one that many here have. I started out thinking solely in terms of miles for redemptions, but because I'm in a decent financial position in my life, I've discovered that using miles to upgrade a seat paid for in cash may be my best option--thus, I've learned my strategy is not necessarily flying for free but instead taking the money I was already spending and getting a better class of travel. My best option is not necessarily your best option, however (and would be a terrible strategy for a young person in an entry-level job who might simply be looking for cheap or free flights so he can visit his girlfriend 2000 miles away), which is why you have to do the research to see what works best for you.

Saying that 100K in US, 100K in UA, and 100K in AA doesn't add up to 300K airline miles is similar to saying that $100 and £100 and €100 don't add up to something around $400 (using rough currency exchange numbers here)--the airline miles may be in different currencies, but they each have value. I hope you're here to get more information to help you find out that value for you.