There are literally thousands of Tarot decks out there to choose from, and more being released every year! How do you know which deck is right for you?
As I've already mentioned, make sure when you look for decks you are looking at a Tarot deck. Other decks are called Oracle cards. These cards can be any number and are based on the author's own dreams or experiences. For this reason, some Oracle decks may not work for you. Tarot cards are based more on what is called the collective consciousness and are universal to more people. That being said, there are still decks that will resonate with you more than others.
One of the best ways to figure out what deck you may like is to go to http://www.aeclectic.com/ and look through the published decks. Many decks are based on the Waite-Smith deck (it is still referred to as Rider-Waite at the website for familiarity's sake), so that is a good place to start. With this one deck you can familiarize yourself with what you may see in many other decks.
Most decks are the same size and the quality of the paper is pretty standard, so these aren't usually issues. If the deck is a different size, it will almost always mention this. There are round decks, large decks, and mini-decks as well.
The first thing to look at in choosing a deck is the artwork. All of the pip cards should have actual artwork - not just a couple of swords or coins on the card. Decks with characters on every card are much more useful in readings than those that only have the number of items on the card.
Things can get tricky when you are looking at the artwork. You may be drawn to the beauty or even "cuteness" of a particular deck, especially if it is a themed deck. One thing you need to remember is that decks may be used for readings, or they may be for artwork. For your first deck you want to pick a deck that you can use for readings.
I don't recommend themed decks for your first deck unless you are sure it really speaks to you. There are decks based on fairies (feys), dragons, vampires, angels, kittens, pirates, and even gnomes. While many of them contain gorgeous artwork, they may not always be the best for readings. I tried to use a fairy deck for some time, but found that items kept moving or even disappearing around my home. Fairies are very mischeivious little buggars, and I have since resigned them to the garage (and yes, things went back to normal, at least inside my house). Some people have much better luck working with them than I seem to. I may try again some day, but only after I take an inventory of where everything is!
Other deck themes include particular legends such as the Arthurian deck. Again, this deck has gorgeous artwork, but can be somewhat limiting because of the story. I do not use this particular deck for reading, though I know two people who use it on occasion.
Some Tarot cards integrate religious paths such as Druidcraft, Witches Tarot, Greenwood Tarot, Thoth Tarot, and Initiary Tarot of the Golden Dawn. There are many others. While many decks gravitate towards a Pagan or Earth-based theme, there are many that do not. Master Tarot, for example, is based on the life of Christianity's Jesus. You can also browse decks on topics such as gay/lesbian.
Aeclectic Tarot has a feature which lets you review all of their decks in alphabetical order. I have done this in the past, and it can take a lot of time over many days. It is very fun, and extremely interesting. Make sure if you use this feature that you look to see whether the deck is a Tarot or Oracle deck.
While you are looking at cards, you will definitely get a variety of opinions and see different reviews of them. Even if a deck is very highly regarded, if it does not speak to you, choose a different deck to begin with. You can always go back and pick up another deck later. The Thoth Tarot gets 5 stars, is a best-seller, and gets tons of positive reviews every year. I eventually bought one, but I do not ever use it -it just does not resonate with me. You have to find something that works for you. One of my favorite decks consistently gets slammed in reviews and low ratings. I don't care - it works for me.
If there is a store near you where you can look at the decks, that is your best option. Actually picking the cards up and holding them will help you decide if that deck really resonates with you. Once you do a spread or two, you will know for sure whether or not you can use that deck. If you can't find a store where you can do this, just go with your gut and how you feel about a particular deck. Remember to think about the difference between "art" and "reading" decks, and you should be able to pick out your first deck just fine!
If you are still really having difficulty deciding on a deck, I recommend the Gilded Tarot or the beginner's set or Legacy of the Divine Tarot, both by Ciro Marchetti.