I found a good personal trainer to be very helpful. Honestly, I would go with someone certified. It doesn't necessarily mean that you are immune from someone who gets you injured or otherwise harmed because they're incompetent, but it reduces the likelihood. I agree with Gany that if you already belong to a gym, start there! Many times you can also do small group personal training to see if you like it. I've done both with the same trainer once I found someone who was an awesome fit for me, and to be honest I preferred the small group and not just because of the much lower cost.
I do think you should examine your goals for getting a trainer. Why do you want one?
*Do you want different accountability than what you've been trying?
*Do you need extra help with your particular circumstance (once I'd lost 80 lbs but still had around 40 to go, I wanted to find someone who'd been on a similar journey that I could bounce ideas and motivation off of. I found a great trainer who was a little older than me--I was in my mid-30s at that time--but who had started her weight loss in her 30s and had kept it off for 10+ years without using a product. She'd also done her weight loss post abdominal surgery and childbirth. And we really clicked personally, she's been in my corner since even though my health is a flaming dumpster fire right now.)
*Do you need to have things changed up but you don't want to/get confused with the resources out there so you want someone else to put together a program for you?
*Do you have special health concerns that you might need some extra support with if you are new to your exercise program? (I have hypermobile joints. I needed someone to teach me what normal range of motion was, and to spot me and help me come up with ways to be able to perceive where I should stop, because my joints would keep going and I could seriously injure myself otherwise).
Those are all pretty solid reasons to get a trainer (one on one or small group). Some trainers will also have additional certifications (or maybe the other way around); my registered dietician that helped me was also a certified personal trainer, but I didn't use those services with her, just the nutrition/eating program services.
So it really depends on what you want.
I didn't look for a trainer until I was frustrated with my triathlon time training, I was hitting a motivational plateau, and I wanted someone who could help me think outside the box.