Buy something used and inexpensive for a starter bike.
Look for something old but not too old. You don't want something so old that it is a junker, but not something so new that you'll feel bad if you lay it down.
Look for something in the 500 - 800 CC engine size. This will be powerful enough so you don't get bored too soon, but not so fast that you will be afraid of it.
Look for a standard or cruiser bike for your first bike. They are more forgiving. Stay away from the sport bikes as they require more skill. Also used sport bikes tend to be more abused.
Learn on this bike. Make your mistakes on it. Would you rather make your mistakes on an older bike or your nice shiny expensive new bike?
Take a motorcycle safety course.
After you have been riding a while and gain some experience, you can sell your starter bike and get the one you want. If maintained properly, you won't lose too much money on your starter bike. Put the money from the sale towards the bike you really want. By this time, you will have figured out what style bike best suits your riding style and needs.
If you settle on a sport bike for your next bike, avoid the used ones. As I said before, these bikes tend to be more abused. So if considering a sport bike, consider something new.
Have fun - Ride safe