I was in the same boat as you, however, going the opposite direction a year ago.
I was looking to LOSE weight and thought about seeking advice from a nutrition specialist and trainer however, realized there are many resources available on the Internet to help meet your goals.
Put in the time and effort to read up on various links/resources and come to your own conclusions. The only major difference in doing it yourself as opposed to having a nutrition specialist is that they monitor/track your progress and do the planning for you. There isn't much of a benefit in doing that and besides, what fun is there when somebody else is doing the work FOR you?
Any way, my own research helped and to date, I've lost 40lbs through proper nutrition/meals and regular exercise. I started off calorie counting but that really sucked. Eventually, I just figured out what I could and couldn't heat, how much of it I could eat, and when. In the end, it has so far worked for me. The guys here in the gym/lifting threads are probably a good resource to start here on RS for some inspiration and then maybe go off on your own.