Welcome to the T&TH site. It's always good to hear of another hunter who has discovered this sport. You probably have already been warned by your friend that it is an addiction. Once you've taken the step...there's no going back and no known antidote or cure.
You've been given a lot of good advice about learning to use a mouth call. For some folks it's pretty easy but for others, most of us, it just takes a while. With persistence and practice I'm sure you'll do just fine.
There are a lot of folks who use a Mossberg 500 for their turkey gun with great success. That is a good reliable gun. I'd recommend, that to maximize the performance of your gun/choke/shotshell, that you "deep clean" your barrel. You can find an article, "Cleaning a Shotgun Barrel"
with some guidelines at http://www.allaboutshooting.com
There are a number of good to excellent turkey chokes on the market. Lots of folks have a favorite or one that has performed well for them. I've had great results with the Mossberg 500 and the SSX Turkey Choke http://allaboutshooting.com/product_info.php?cPath=23&products_id=76
That choke has great internal geometry, is rated for all lead and "hard" turkey loads and is very reasonably priced for a premium quality turkey choke.
I've also had good results with most of the 3" HV shells but always see better patterns with #6 shot. One really good load is the 3" Winchester HV (Black Shell) with 1.75 oz. of #6 lead shot.
Again, it's good to have you here. I know you'll find this to be a friendly place with lots of good folks who are happy to be of help.
Best of luck this spring and please let us know about your hunts.
P.S. Thank you all for the kind words and recommendations. I'll do my best to be of help.