Broad question but it all depends:
Have you rode a dirt bike before at all? Are you responsible with something that is quick?
I started on a 700 but I knew what I was getting myself into. If you get a sport bike you just need to remember you're the fastest thing on the road around you most the time. (If said bike is 600+)
Take your BRC or MSF course and get the fundementals down then go have fun, and always wear gear! You dont want to loose your skin.
Also remember that people now adays dont pay a lot of attention while driving so always be on the defense!