Set a goal. Decide whether you're interested in collecting paperbacks by particular authors or cover artists, TV tie-in novels, a specific genre or era or a single publishing line, such as the two-books-in-one Ace Double books.
Research your subject. If you're interested in a 1950s line of Agatha Christie paperbacks, or 1960s reprints of the Doc Savage pulp novels, learn how many books there were in the series and what you can expect to pay for them.
Learn the grading system. Paperbacks are graded from "near mint" books that look brand new down through "fine" to "good" - torn, rolled up and possibly stained, but still readable - to "poor." The lower the rating, the less status as a collectible the book has.
Draw up a wish list showing which paperbacks you want most and what your budget is for collecting them. At some point, you may have to choose between paying top dollar for one rare book or buying several cheaper paperbacks.
Hunt for books in your specialty. Garage sales, flea markets, estate sales and used book stores in your community may have some bargains, plus the Internet offers many used-book sites, such as alibris.com or abebooks.com.