I would absolutely recommend that you start off by playing in a few random draw doubles events. Jumping straight from casual play into a PDGA tournament can be jarring, given the focus on rules, etiquette, slower pace of play, etc. A weekly, local, random draw doubles tournament is a good go-between from casual to true tournament play. At doubles, the atmosphere is typically more relaxed and people are much happier to help out.
Typically, people at doubles are welcoming to beginners. They're glad to help out and will encourage you to keep coming back. Doubles is also one of the best ways to learn and get better. Occasionally there's a stick-in-the mud who is unhappy to be stuck with a newbie, but just ignore that guy; most of us are happy that you're there. The entry fee is only $5 or $7 ... no reason to get upset with that much money on the line. If I was paying $40 to get in and get paired with a total newbie, then I'd get a little grumpy.
Generally speaking, a random doubles tournament will be a captain's choice/best shot format. Both of you will tee, you'll pick the better lie, and both of you will throw from there. If you get partnered with a good player and end up taking most of his shots, don't worry about it.
A lot of the time doubles players will try to alternate shots. I tee first on even holes, you tee first on odd holes. If we end up taking my drive, you step up and take the first shot on the approach/putt. That's kind of the "default" behavior in doubles. However, when I get paired up with a newbie, I typically just tell him to go first every time. I let him go first to take pressure off him and put it on myself. I try to be confident in him ("go ahead and sink that putt, I'll be back here") but always shrug it off if he messes up ("no worries, I think I can back you up on that").
Show up half an hour early, tell the TD that it's your first time out at doubles, sit back, try to meet some people, have fun.