This is a great article about the 10 things a good personal trainer will do for you. There are a lot of folks out there claiming to be fitness and diet experts when they're no more knowledgeable than we are. Those are the people that get you killed. If you have the stomach for it, Google personal trainer horror stories. If not, just take my word for it.
If you can't afford a personal trainer, surely you have a friend who will work out with you. Having a running buddy has helped me so much in the past (thanks Red!!!). We pushed each other when the other didn't want to run, we made time for running when we'd rather be watching tv or shopping, and we encouraged each other to sign up for races. If you don't have someone who will run or workout with you, try finding someone online. Meetup.com is a great resource for meeting people in general, and local running clubs are always looking for more members.