New to marathons!

I have been wanting to run a marathon, and i found the right cause for me. In lieu of my cousins, i decided to run the half marathon for autism, Sept 20th. that gives me just over 3 months.

I just needed to know if this is impossible, and if not(impossible) then I'd like to know some tips from you experienced ones. I sooooo appreciate your responses in advanced. Im beyond excited to accomplish something that means so much to me =)

~Bri