Not every comic is worth a lot of money.  The characters, condition, age, rarity and importance and the main keys. 

Characters

Mainstream characters (think about the characters that have had blockbuster movies) demand the highest prices.  The simple reason is that more people have heard of them and more people want issues with them

Condition

The better the condition, the higher the price.  Comics are graded (in best condition to worst) Mint (MT), Near-Mint (NM), Very Fine (VF),Fine (FN), Very Good (VG), Good (G), Fine (FN), Poor (P).  There are several grading companies (PGX, CGC) that professionally grade the issues for around $20 (these sell for a premium).  If you want to try and grade the issues yourself, do a google search on "comic grading" and go from there.

Age

Anything before 1970 will sell.  Obviously, it adds to the scarcity if it is older.  Very new issues tend to do well, too since collectors want to fill in holes in their collections or stock up on issues that they think will be hot.

Rarity

This is a supply and demand example.  As an example, most issues from the early 90s were overprinted (some with over 1,500,000 ccopies) while most newer issues have print runs well under 100,000.

Importance

First appreance, #1 issues, hot writers, hot artists, deaths are a few examples.

Sites to check for values

The three easiest (and free) ways to find the value of a comic would be

  1) check past sales on ebay

  2) comicpriceguide.com has prices and a few books for sale

  3) AtomicAvenue.com is a hot site with values and users can post their comics without paying FEEBAY'S posting fees.  Because of this, there is a huge inventory and the prices (and shipping) are reasonable.