hi there and welcome to the board.
i dont think you will have much trouble with the certification part- that is pretty easy. make sure you understand everything and WHY everything is. Read the manual front to back and just chill out and have fun at the pool classes. the certification part is really easy if you have the right attitude.
as for the gear buying, i say hold off on all the really expensive stuff and start off with a mask, snorkel, and a pair of fins.
make sure the mask fits your face really well (VERY WELL, you dont want a leak.) do this at the shop by putting the mask on your face and takking a big sniff in with your nose. see if it holds suction. if you are going to use it for your pool classes, make sure you scrub off all the silicone film that's on the glass. make sure you get it ALL. use toothpaste- its a generally gentle cleaner and it will do the job. the silicone film and the cold water make the mask fog up VERY quickly, and it won't be fun.
i got a pair of open heel fins, which i really like since i think they give me more control of my fins. you can also get full foot fins, which i think they happen to have most in those rental places in the carribean and FL. with open heel you must also buy a pair of boots- also a plus if you are going to do some cold diving. make sure everything fits you feet snugly, and they are not too tight lest your feet cramp up when you try to use them. the boots- i usually wear a size thirteen shoe, and i got a size 12 boot simply beacuse i thought they were more snug. i would reccomend personally getting a size smaller- whatever rubbs your buddha.
snorkels- make sure they dont hang in your face and get in the way. some of them have wierd little tops on them that prevent water from splashing in- a plus if you are going to do some ocean diving. nobody likes a mouthful of salty water. i went without- again whatever rubbs your buddha.
i would say holding off on everything else and renting until you find whats most comfortable for you. as applies to everything, there is nothing worse than being cold, wet and uncomfortable so wait and test out all that you can to see what is best for you.
hope i am helpful.