When starting out, I'd recommend running a minimum amount of gas inside the suit, just enough to offset squeeze. A smaller drysuit bubble makes it easier to control ascents and achieve horizontal trim. It will take some time to become comfortable managing two separate buoyancy devices (BCD and drysuit). As others have suggested, practicing in the shallows is a great learning exercise for buoyancy control. During ascent, stay ahead of the expanding bubble inside the suit by venting early and often. Your instructor will show you how to do this. In the class, you'll learn how to deal with runaway feet-first ascents, stuck inflater valves, and blocked exhaust valves. Ask lots of questions about drysuit maintenance. Know that latex seals and your dry zipper (wax both inner and outer teeth) require extra care. It's helpful to have someone assist with trimming your neck and wrist seals if you have the popular latex variety.
As you become more comfortable operating the drysuit, feel free to experiment with running different amounts of gas inside the suit. This will give you an appreciation for the additional freedom of movement that comes with a larger drysuit bubble. Don't overdo it, though, and impersonate the Michelin man. Be forewarned that certain drysuit + undergarment combinations may have issues with exhausting gas expeditiously. With the relatively shallow dives that I've been doing recently (max. depth 60 fsw), I've found that, for my combination of shell drysuit + undergarment, adding enough gas to offset squeeze is just about enough for adequate buoyancy control. This simplifies ascents since I only have to dump gas from my drysuit.
Do a weight check with your drysuit + undergarment combo. Understand that how much gas you're running inside your suit will factor into determining your minimum weighting requirements.
Be patient. To really feel comfortable in the drysuit, you'll need to do 50+ dives with it. At that point, adding/dumping gas will become second nature.
From what I've seen, experienced wetsuit divers who have good buoyancy control and excellent horizontal trim tend to pick up drysuit diving the fastest. Good luck with your new suit!
