First off, there are many kinds of trainers. They can be 'lifers' living the lifestyle for their entire existence, they can be certified, they can be assigned to you at the gym, your brother, or your neighbor.
Look around and take in who around you is healthy and living the life you wish to live. They could be a great trainer for you.
If you have the money to hire a professional - GO FOR IT! They have the knowledge and passion to get you started and keep you on track to a great, healthy life.
Take stock of your goals and weaknesses and look for the trainer that will most complement you. Choose wisely. This is a very personal experience that is one-on-one and very intimate. If you are at the gym, hang out on the elliptical for a few days and watch the trainers. Who is getting results and motivating their clients? Who is gentle? Who is a whip cracker? Who is a talker? Who is aggressive, passive, nurturing and so on....
None of these are bad traits in training but you need to hook up with a trainer that will inspire you!
I can't afford a trainer!
If you can't afford a full-time trainer sign up for two or three sessions to learn how to work out properly and design a routine for the next several months. Be clear with them about your goals and request guidance in formulating a training program that you will proceed with. Learning how to do each exercise for maximum benefit and safety is invaluable. Learning balanced, systematic routines for the different muscle groups is powerful information as well.
A tiny, big secret...
Gyms that employ trainers hire them to help you. If they are just standing around don't hesitate to ask them about proper techniques and if you are doing the exercises correctly. That's what they're paid for!