The easiest and quickest way is to call your local comic shops and find out if they buy used books, and if so, when their buyer is in. Then pack up what you have and see what they'll give you for it. Other methods which are more of a pain and less likely to pan out- ebay, yard sales, craigslist, going to conventions to meet dealers and see if they want to buy your collection as a lot, etc.
Price guides are sort of iffy- yes, a comic might be "worth" $20, but if you can't find a buyer for it...well, it's not worth that much other than what it's worth to you personally. And you won't usually get the listed value of the book- that's what the store will try to sell it for if they buy it from you.
You might get that price on ebay, or not- it's tougher when the buyer can't see the book and really evaluate what shape it's in, and the quality grading scale is somewhat subjective- reading a book once or twice can significantly lower or destroy the value, believe it or not. If it's an old or rare title, condition isn't as important, but it still counts. A comic that looks nice could actually be in "bad" condition in collectible terms, unfortunately. Most people who collect seriously read them maybe once, carefully, tape them in a mylar bag with an acid free board, and seal them away in an acid-free storage box and never touch them again.
Be prepared to get less than cover price for most issues, especially newer books, if they even take them at all.
DH has a huge collection that we're trying to thin out, not because we need the money, but because we're out of room. Twice a month he brings over a box of books and more often than not, gets about $20 in cash or $30 or so in store credit for a few of the books and gets handed back a stack of stuff they don't want. Averaged out, it's probably about 75 cents or less a book.
We take the store credit, since he still buys books and that saves him money, he's basically trading books he doesn't want for books that he does. We figure it's a good enough deal, as his hobby pays for itself and I slowly get my closet space back.
We could probably make a bit more on ebay, but when you factor in the time, hassle, and obnoxiousness of dealing with comic book geeks

it's just not worth it. (I am a comic book geek. I'm allowed to say that!)
The speculation in the comic industry really killed the market for collectible comics- everything came out with a foil cover, a limited edition, a special bag, you name it. Basically, what happened in the mid-90's or so is that people were just buying comics to resell and as a result, publishers flooded the market with comics intended to be "valuable" that really aren't, and now most books have such a huge print run that they're not likely to become rare enough to have much value.
Sorry, I'm a comic book geek myself, so this might have been more info that you wanted. Personally, I buy books I like and want to read. If they turn out to be worth something, great, if not, I still have some good books!