you don't NEED an instructer but it wouldn't hurt as long as the instructer was knowledgable.
as far as worrying about others in the park...i know at that age everyone is self conscious but theres really nothing to be scared of. as long as you do your best to get out of the way when you bail or fall then you'll be fine. i doubt anyone will give you a hard time especially being a girl and anyone who does bother you isn't worth listening to. there are tons of idiots who think they're cool that hang out in the park who are always much worse than an single individual who's just trying to get better. as long as your being respectful and following the park etiquette, no one will probably notice you.
remember to start small and move on to larger features once you get comfortable.
as far as competing, you probably wont ever be winning gold at X-games but that doesn't mean you cant compete on a smaller scale. your only 14 so your still really young. i wouldnt stress it though. just compete with yourself every day to try and get better and have fun!