Congratulations!

People probably get negative because of your age - what I mean by that is they think they know better, and that you have to listen to them merely because you are 20. They would probably never question me cos I'm 33 and would give them an earful. The issue won't be that you "can't do it", just that the whole world must know better than you. Get ready to hear opinions from everyone in the entire universe on your situation. The checkout chick, the nanna behind you, your family and friends - this however happens to all of us regardless of age. Just be ready for it!
Tap into all available resources - BB (of course), support groups (not as nerdy as they sound), financial planners and positive people. Find a student midwife or Doula. If Centrelink give you a hard time let me know, I can help you if needed.
I would just like to say I have the pleasure of knowing 2 quite young mums with twins and I take my hat off to them, they are both doing a fabulous job.