Hey there, and welcome to the forum! It seems like a lot of people are thrust into the hobby when they stumble upon a massive cache of old posters... I'm surprised by how common this is, because working at a theatre, I can say it's not long before posters are destroyed or taken home by the staff. If we had a storage unit for all of our old posters, it would only be the worst-of-the worst that survived weeks if not months of scavenging.
You'll meet a lot of people here who want to A) Buy your posters and B) can help you determine the value of your posters. I think the first bit of advice you'll receive that's pretty unanimous is not to sell your lot to a retail site like MoviePoster.com. You can make far more money selling them individually on eBay or to your fellow collectors. Plus, you never know if you have a gem in there. Dark Knight posters, for example, are usually $50+ a piece. So you might have a tube sitting in your pile worth $150!
The smartest thing to do is to take the time to document your entire trove. That way you can weed out the worthless stuff (which there will likely be a lot of) and capitalize on the valuable paper!