Dan:
Glad to hear you're taking the plunge - I definitely reccommend a personal trainer if you're starting out seriously for the first time (or even if you're coming back into exercising after a long hiatus). They can get you set up with a routine, make sure you're doing it correctly and safely, and in a lot of cases can set you up with things like diet plans and whatnot. They definitely run a little on the expensive side, but if you look at it as an investment and pay up front it's a little easier to swallow.
Some tips and/or things that worked for me:
1.) Going along with the trainer thing - I find that having someone instruct/coach/yell at me gives me 2-3x better results than just working out on my own. This doesn't mean you have to have a private instructor - look for things like boot camps and group fitness to get the same experience at a lower price (there's a new-ish chain of gyms called Orange Theory Fitness that does this pretty well).
2.) Get some workout videos and some gear to use at home for when winter hits or for days where you just don't get a chance to go to the gym. P90X, as cliche as it is, is a fantastic workout and has lots of variety. I haven't done Insanity but I've heard good things about that as well. Pick up some basic gym supplies - resistance bands, a yoga mat, some weights, maybe a medicine ball or two...there's a lot you can do at home with that stuff.
3.) Beware of the plateau - when you start out, you'll lose a ton of weight and see crazy results fast...then over time your workout regimen will become a routine, and visible results will come a lot slower (or stop altogether). If you reach this "plateau" you might want to try something new, alter muscle groups on different days, buy a couple training sessions, join a spin class...etc...
4.) Diet is crazy-important to weight loss and maintenance - almost more important than going to the gym, in my opinion. Drink a ton of water, as everyone has said. Cut out soda as much as you can (that includes Diet Soda, which I've personally scaled back on a ton...and I love diet soda like no other). Think in terms of calories-input vs. calories-output. If you eat that value meal, you've just negated like 2 days worth of workout in some cases.
Good luck!