First, research Enid Blyton's books. Her career spanned over 50 years with varying degrees of popularity, and there is a lot to learn about her literature. Many of the series books she wrote can be extremely valuable and expensive, but there are other works of hers that someone just starting out can acquire without too much of an investment.
Learn about a book's condition before you purchase anything. The value of a book depends on its rarity and the desire the public has to own it, but also its condition. An extremely rare book in poor condition can be worth much less than a lesser-known book in excellent condition. The Independent Online Booksellers Association, as well as other sites, maintains a page on book condition.
Decide which series of books or type of books you want to collect. No one can collect all of Blyton's books without spending literally years and hundreds of thousands of dollars, but anyone can start by picking a niche.
Search for the books in many places. You can purchase books from Abebooks.com, Alibris.com or on eBay, plus from some independent bookstores. Many collectors, though, love the search for books as much as the collecting itself. A book search becomes a treasure hunt. Search estate sales, yard sales, sales in private homes and library sales. Treasures come up in odd places, and you may be surprised to discover a very nice collectible one day for pennies on the dollar.
Store your book collection carefully. Of course you want to show it off, but protect the volumes as well. Keep them dust-free and out of the sunlight. Never stack books with anything else on them, and keep them away from small children and errant liquids of all kinds. Purchase Mylar covers to cover the book and further protect its dust jacket.