First off, you have your passport, if not, then get one and then start planning, because you just can't travel any longer without one. Do you need a visa? Countries that need a visa to enter Mexico, those that don't .
Yes, you have your passport, you've then maybe chosen an area in the bay area or the town that is best for your interests, you now need to choose your accommodation.
You should choose your hotel based on budget and interests, beachfront for those that enjoy beaches, swimming and sea activities, close to shopping options or not, close to the attractions, romantic or boisterous and so on. This will require a bit of research. Use our hotel maps by zone to find the hotels that are best for you.
Buy your plane ticket or package and plan your perfect trip, make a list of what you'd like to see, visit, experience and enjoy and then find out how you can get it done.
What to pack
Depending on the season you'll pack light or very light summer clothes, a light sweater or cotton jacket for nights if you are the chilly type, but otherwise just the summer stuff. Maybe a fancy dress, pants, jacket if you are going to go to the "chic" restaurants and nightclubs, plus your comfortable tennis shoes for long walks and tours, flipflops are very common "gringo" trait. The rest of the list should at least include: money, debit/credit cards, sunblock, swimming clothes, insect repellent for your tours.
Passport with visa if necessary
Health insurance card
Money & credit cards
Your airplane reservation and contact phone numbers
Sunglasses (prescriptions too)
Cellphone (contact your provider beforehand if you're going to use it in Mexico) and charger
Any important prescription medicine
Reading material for the trip
If you forget anything you can get these products in town at one of the local supermarkets or Walmart & Sam's Club. See our supermarket list.