The Learntarot.com site is excellent.
Pick the STYLE of deck you want to learn. They break down into three major groups–
Rider Waite decks
Each reads a different way. Pick one and learn that type (the learntarot site is for the Rider Waite). Each card has a basic theme, and it hasl 6 or 7 meanings in reverse. The sequence also has a theme, which helps in learning the cards.
When you lay them in a spread, the position the cards are laid in also has meaning, which gives depth, but for the actual cards themselves, learn each one separately then how each compares to the card on both sides of it in the sequence it falls in in the deck.