Always choose a tarot deck that makes you feel at ease and secure, but also inspired. Since you will be investing a lot of time in your cards, you don't want to pick a deck that strikes you as unpleasant and/or low in energy. Later, you may seek out unusual decks for the challenges and insights they offer, but it's better to start with one that attracts you. If a certain deck calls out to you, go intuitively with that one!
There is no official tarot deck. Decks come in many different forms, but the "standard" deck has got 78 cards with 22 Major Arcana and 56 Minor Arcana cards divided into 4 suits. Most of the decks are built on this model. You should probably stick with a standard deck to start so that you are familiar with the most common format.
Many decks are oriented around a theme. This is especially true of modern decks. Typically, the images, the names of the suits and the court card figures reflect this theme. If you choose a deck with a theme, be sure it is one that suits you and that has lasting appeal.
The Rider-Waite is probably the most common deck in the United States, and many tarot decks are based on it as well. Cards in these decks often have the same subject matter as the Rider-Waite, but are drawn with a different style and artwork. The Universal Waite is essentially a copy of the Rider-Waite, but with softer colors and less contrast. The Albano-Waite has bright, unusual coloration. Here's a side-by-side comparison of some cards from the two decks.
In some tarot decks, the pip cards, or numbered suit cards, all have unique picture scenes. In other decks, these cards simply show the suit symbol repeated the appropriate number of times (similar to regular playing cards). Some people like these symbolic decks, but for learning and memorization, it is often easier to have the pictures.
Some newer tarot decks have been created in the spirit of light-hearted fun. Two examples are the Halloween Tarot and the Silicon Valley Tarot. These decks are amusing, but not the best choices for deeper, more thoughtful tarot work.