Well....for starters timidity will get you nowhere. You really need to be a leader, have rules and set boundaries.
I can't stand horses I have to tip toe around or who will just bowl over me. There was a great thread started about ground work. If your new to working with young horses I would strongly suggest working with an experienced trainer to help you along.
A good place to start would be having the ability to touch her anywhere, get her to pay attention to you and respect your SPACE. Once thats done exposure exposure exposure. Cars, bikes, trucks, four wheelers, sack her out, freaky noises, music, screaming kids :)
~Horses aren't a hobby, they are a lifestyle. Website