Right, tub goes in first. It has to be levelled and shimmed first, then the drywall can go on top afterward (or set in a foundation first). Otherwise you'll be cutting and finalizing tidbits of drywall here and there and... and you know how much fun it is cutting say 1/8" of drywall
I don't think it has to be done by a certified plumber, but is on the advanced side and not a bad idea. I'm most familiar with cast iron tubs, I just hate fiberglass & plastic. Back in the day you used to rest the tub on a lip (usually just furring) and then shim it to level. Being level is very important for proper draining. The hardware was soldered in place so you often had to get it hot enough to solder but not ruin it. And, sometimes had to solder some of the plumbing beforehand but not all (else tub wouldn't fit). The innards are delicate, so you had to be careful not only of getting the assembly too hot but also not using too much solder. Then had to worry about how far the controls stick out there isn't especially a lot of room for error. Then the assembly can't handle much debri as debris can ruin the assembly a plumber will flush the line first before final assembly. And plumbers know very well to make sure the showerhead lines up with the controls which line up with spout (it's actually pretty easy to screw that up) and adjust the plunger depth.
So it wasn't an easy task, and really still isn't. Today tubs (cast iron) have feet and you level them by putting shims under the feet. The controls most often take threaded fittings so there is no soldering the assembly (no risk of having to get it just hot enough nor putting too much solder). The controls now come with a plastic face guide to let you know exactly how deep the controls have to be countersunk from the face. Still have to worry about scraps & solder interferring with the innards but the assembly has a screen now (removed before final assembly). Adjusting the plunger is pretty easy.
My $0.02 I would say you're fine getting an apprentice or someone who's done them before (the more the better), someone licensed you're pretty certain to have a good experience, I would just make sure this isn't the persons first install it's pretty involved.
bath tub installation help
Reason being you need to have the dry wall and the tiles to overlap the tub to prevent water from running out through the back wall.If you look close at the tub you should notice that their is a lip around the tub 
thats where the wall will some what sit on.I hope this help
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