Sue, we have done it both ways and luckily, we had no problem with selling our first house on our own. We went to Staples to get the forms and had our attorney look through them after we had an offer to make sure the transaction looked ok.
Having said that, we used an agent for our second house and not worrying about the paper work was nice. If you are in no hurry, try both ways, do a month by owner on your own and if you get no results, list with an agent.
It also depends on how fast you want to sell your house, as Jim said. With a realtor you will get a much wider audience.