The best way to get into dealing books is to jump right in with two feet: scour your bookshelves and gather up a pile of books you are willing to get rid of. This pile (small or large) will give you a chance to experiment with judging a books' value and desirability without having to risk sinking money into purchasing stock.
Set up a dedicated email account and establish yourself with accounts on the three most basic websites: Amazon, Ebay and PayPal. Before actually selling with these websites you'll want to practice using them—surf through books similar to the type you are selling and look how they're described and priced. What are people actually bidding on? What is the market glutted with? You'll probably want to go the final step and actually order a book from both Ebay and Amazon, just so you can be sure you know how the system operates, the better you know the customer's point of view, the more you can ensure that they have satisfaction from you.
Now you're ready to start posting books. Be conservative in your descriptions and cautious in your packaging with your first packages: you haven't got any reputation online to back you up and you want to ensure that your early reviews are good ones.
With time, you'll want to add to your stock: I suggest letting loved ones know that you're in the market to take any old books off their hands (perhaps offering them a cut of any proceeds) as well as any old packing material. You should start hording packing material as quickly as possible.
As you start to wear down this free seed-stock of books, you can begin to actually start book-hunting in earnest. I will have more on this in a future post, but good places to start are thrift shops and library sales. Experiment, but never pay more than $1.50 for a book until you get the hang of it.
Congratulations, you're a book dealer!