Here’s another idea for you as well. Get a free account at http://www.meetup.com and join all the MTB groups that interest you. You will get email notices when rides are planned and you choose which ones you might want to join in on. It’s a great way to meet new people and learn new trails. They will specifically say if it’s a beginner ride, how far or fast they plan on going, etc…There are many, many biking and outdoor enthusiast groups in Phoenix. You can join one or five or fifty. There are meet up groups for everything under the sun from yoga to child play dates to old car junkies. Here is the mountain biking link: https://www.meetup.com/find/?allMeetups=false&keywords=Mountain+Biking&radius=50&userFreeform=Phoenix&mcId=c85001&mcName=Phoenix%2C+AZ&sort=default
You don’t need full suspension in Phoenix to start out. Phoenix has tons of bike shops and many of them have “demo days” where you can ride the bikes they sell for free. That way you can check out many different bikes, brands and models to see what suits your style, price range and size. You could scour Craigslist for a decent bike but it all depends on how much you’re willing to spend.