How much is an Amazing Spider-man #33 worth today?
I collect comic books and I have this one. It's from 1966 and contains all its pages including the adds. You can see how the cover looks like if you type in the name and number on a search engine.
What it is worth depends on what condition it is in. If there are no creases or tears on the cover, the colors are bright, maybe there's some gloss left on the cover, the inside paper is white, soft, and supple, and the inks on the interior pages are bright, there is no writing, water damage, dirt, or anything else defacing the comic anywhere then it might fetch $50 or more (again depending on exactly how good the condition is). If it's a well-read, beat up copy, and the paper is yellowing with age, you might only get around $20 for it. That particular issue is quite famous - it is the finale in a three part Doctor Octopus story, and has the often quoted/referred to panel of Spider-man hauling all that machinery off himself so that he can go get help for Aunt May, so it does get a little more than some of the issues either side of it. And of course it is a Steve Ditko classic, towards the end of his defining run on Spider-man. But what your comic is worth really does boil down to condition. In immaculate, near mint shape it would fetch hundreds, especially if it was professionally graded ('slabbed'). If it's been read over and over, the corners are dog-eared, there's tears and creases everywhere, you're looking at much, much less.
If you think you have a copy in exceptionally good condition (and you might want to take it to a comic book store and ask somebody if you're not sure how good it is), it might be worth spending the necessary to get it 'slabbed' - here's the link to the CGC website - your portal to professional comic grading: http://www.cgccomics.com/
But it is only worth spending the money if it is an exceptional copy, which will then fetch a high price at auction, more than if it wasn't slabbed, usually, enough to make it worth forking out the dough for the grading.
But if it's a run-of-the-mill mediocre condition copy, and you want to sell it, make some clear scans of it and slap it on eBay.