Determining Stamp Values depends highly on what types of stamps you're talking about. The title hints at it but there are very different steps to figure out what your stamps are worth.
The current postage rate is also an important consideration when valuing modern mint (unused) stamps so we'll cover that as well.
This page touches on all aspects briefly and you can click the embedded links for a page with much more detail on each topic.
Stamp Collecting Values - Start here if you have boxes, albums and all kinds of mysterious stuff
Postage Stamp Prices - Check here for info on how to determine prices for relatively common stamps using stamp catalogs and online comparisons
Postage Stamp Value - to learn more about how condition and centering affect what individual stamps are worth head to this page. Condition is everything!
Non-denominated Postage Stamp Values - This page will help figure out the denomination of modern stamps that either don't have any amount listed on the stamp or have a letter (A through H) displayed in the design.
Discount Postage Stamp Cost - Save money buying discount postage for use on mail. The over supply of modern stamps presents an opportunity!
US Postage Rates - click here for the current letter rates as charged by the US Post Office
Package US Postage Stamp Price - The latest Priority Mail and Express Mail rates and stamps are discussed
I know that is a ton of detailed pages but that is the way of stamp collecting.
The truth is that the hobby requires such specialized knowledge and is so fragmented by different collectors and users of stamps that you really need to identify what you have before you can ever start to think about what they're worth.
It takes many years or decades to accumulate enough knowledge and market experience to accurately evaluate how much a stamp collection is worth.
With that in mind, the pages above provide helpful tips and thumb rules to help even the most inexperienced stamp collector get a reasonable idea of what sort of stamp collection you're dealing with before spending precious time and effort researching further in the hopes of hitting it rich.
While the overwhelming majority of stamps are worth little to nothing, there are lots of exceptions and you never know what you have until you check!
While most of the stamps in an average stamp collection falls into the category of US Postage (such as Forever Postage Stamps, Love Stamps and Custom Postage Stamps) there are always finds out there to be made. Some early US Stamps are worth hundreds of dollars and the sky is the limit when you start talking about the truly rare.
The only way to figure it out is to investigate your collection thoroughly and it will take some time, effort and research. Good luck and I hope these pages can act as a helpful guide!