I would begin taking lessons right away. If you are looking for a teacher, here are some links:
http://www.uspta.com/
http://www.ptrtennis.org/
http://www.oscarwegner.com/lucile/MTMCA/Welcome.html
http://localtennislessons.com
https://www.playyourcourt.com
https://onlessons.com
http://www.playtennis.com
http://mytennislessons.com/
If you live in the USA, you should be able to find an instructor near you in one of the three web sites above.
To prepare for tryouts, I would attempt to get in as many practice matches as possible. Hopefully, your teaching pro can help you find someone to play. Group lessons are also good for meeting people in your own skill level.
Do NOT spend $200 for your first racket. Find out what your correct grip size should be [don't use a formula] by hitting with several loaner [demo] rackets of varying grip sizes, until you get one that's so small that the racket turns in your hand after an off-center hit. THEN, go to Costco, Walmart, etc and get two inexpensive rackets [pre-strung] in your correct grip size. Probably less than $100 for two rackets!
Get a nice pair of tennis-specific shoes. Do not skimp on your shoes! They must fit you correctly in width as well as length [finger's width extra inside, above the big toe]. Wear your new tennis shoes around the house or apartment for 3 or 4 days prior to playing in them.
Clothing? Look for comfortable clothes that allow your plenty of *freedom of movement.* You do not need designer clothes worn by Serena, Caroline Wozniaki, etc etc. Wear "normal" clothes that don't stand out. It looks odd to see a beginner dressing like a Wimbledon champion, IMHO. Look at what the other girls are wearing, but if you can't afford them, save your money for lessons and good shoes.
Buy tennis balls by the gross or half gross and a big-box store, not by the can. Usually cheaper that way.
Sportsmanship might not be #1, but it's better than whatever is in 2nd place :-)
Learn the official rules of tennis as well as The Code.
http://www.ustashop.com/ <-- place your mouse pointer over NOVELTY, then click on Books. The 2013 Friend at Court book is now on sale for only $13. Buy it, if you can. FAC is the USTA's #1 rule book. It has everything in it, plus, an index in the back for looking things up.
all the best, hope you have some fun along the way :-) The game is a challenge!