Lots of good info ..../ doing this all on your own ...is the hard way to do it ...and you may develop all kinds of bad habits. Until I finally met someone that could "teach me" the right way to do things, help me with gun fit ....I had a hard time.
But if you have shoulder pain - that gun may not come close to fitting you .../ or there may be some significant fundamental issues on the way you mount the gun ...picking up your head, etc - hard to tell without seeing what you're doing. When you're shooting an "angled comb" gun like the stock on a typical 870 - moving back a little or changing the angle of the comb - will probably make a huge difference. You can do that with stick on comb pads ...different recoil pads .....or even going to an "outfit" ....where you shoot with a fatigue sweater or something under your shooting vest ...
But a good lesson will help you sort all that out hopefully too.
You may have a number of guys that can help you at your local club ...ask around ....sometimes the quietest guys won't help you unless you ask - but they may know the most !! In general, I would avoid the guy that is the loudest ....and has all the answers ....and often tells you, you have the wrong gun ....and just starts to tell you everything he knows .... ( every club has at least 5 of these guys !!! )....
A good lesson from a certified instructor is always a good idea - even for experienced shooters. But you have to find someone that will help you with your goals ...and listen to you .../ not force you into their "program" ....and that's hard to find.
But there are some good DVD's out there too - from big name instructors and shooters.... Sunrise Video - is a good source...
http://sunrisevideo.com/
I've used a number of their DVD's - Bobby Fowler Jr, Blakely, Todd Bender, etc .... but if you're looking for something specific to "Trap" --- I would probably start with Harrison's DVD...