just to echo a ton of good advice and input... the first thing to decide is what kind of riding you intend to do! If you plan to ride in the dirt or sand, you'll want a mtn bike... if you plan to strictly ride on pavement, you'll want a commuter or a road bike...

the one thing I tell people is that it's an investment... much like a car... so ride every single bike you can get your hands on before you buy!!! Buy the best FRAME you can afford... the components can always be upgraded when you get more serious! (notice the use of "when" not "if" )

as to desert riding... if you get a road bike you can get tires that are much better in puncture resistance... I ride Continental DuraSkins because I got tired of changing flats... my last set I got 2000 miles out of!!! that's a decent amount of miles for road tires...

fit is very important... try to find some shops near you that have fitters and they will direct you to the size range you will need... it really varies person to person depending on inseam length and upper body length... I'm 5'7" but I ride a 49cm MEN'S bike because I have short legs and a long torso... other women who are 5'7" may ride a 51cm WSD bike! Get whatever fits YOU best! If you end up with a guys bike the shop can WSD the brakes etc for you so your reach won't be an issue...

good luck with it! OH! And your weight has nothing to do with being able to ride! I have seen riders of every size out doing centuries! Mary is a wonderful example and she has shed much weight since taking up this wonderful hobby... my brother also completed his first century at 320lbs... and is now down to 229!

And last but not least.. where are you in SoCal? I'm in San Diego... if you're nearby let me know... would be fun to do a ride sometime!