I'm a high school and travel team softball coach. I primarily coach the younger ages and JV, because I like working with girls who have the potential and the desire to become good players.
The first thing I look for in a girl ISN'T skill. It's athletic ability of any kind and a willingness to learn. If you've got those two and can demonstrate them to a coach, you've already got one foot in the door.
Since your tryouts are nearly a year away (I assume), you've got a lot of time to prepare. If you're serious about softball, try to find a fastpitch league in your area. Leagues range from teams that play locally on weekends to teams that travel the nation. My local paper lists teams that are looking for players in the sports section on the page with all the box scores and transactions. If you find one, give them a call and see what they're looking for. It's important to find a good fit.
Like others have said, there's some really good DVD's out there that can help. Pick one up! I like the series by Sue Enquist, the former UCLA coach. You can get them all over the place if you do a Yahoo! search for them.
Buy a decent bat, a cheap batting tee and a good glove. Get a couple of softballs and play with them as often as you can. Using the DVD's, work on your swing, your throw and on fielding. You'll need help for some of those, so find a friend, preferably one with experience, to help.
With all of that, don't be disappointed if you don't get a lot of playing time at first. I have a dancer on this year's JV team who came to us with no real softball skills, but she was athletic and had the desire. She sat on the bench for the first few weeks, but got steadily better at practice and kept a great attitude. She started for us in our last game and had two hits and drove in a couple of runs. She won't be sitting for us again!
Good luck and remember, if softball doesn't work out for you, there's a LOT more out there for you! One of my best softball players I ever had the privilege to coach discovered golf in her Junior year and fell in love with it. So don't narrow your focus too much. The important thing you need to do is to have fun!