As with all such tools, it should be used carefully and treated with respect. The amount of good that I have seen accomplished in people's lives because of them is immense. It just depends on how you use them. If you are giving a friend a reading, for example, you don't tell them everything you see.
Sometimes what you see won't help them- it will hinder them due to their obstructed mindset and perception of the information.
If they are in a negative downward spiral and you see that they are going to be there for a bit until they learn something- you have to let them play it out without telling them this fact.
Telling them this fact will just change the little hope they have for things getting better into nothing- they will then based on your reading foresight, give up where as if you tell them what they need to hear like "it's almost over- something all of this was trying to teach you is becoming visible" is much better than "your going to suffer much more before it's over"- make sense?
If you want to learn to read Tarot cards, there's no better way than to simply start reading books on the topic, playing with the cards, and doing readings that at first will be gibberish until you start to define the cards intended "basic" meaning. Later you will give them deeper more personal meanings based on your own interpretations.
These are the encoding of your thought stream that will be translated into imagery. The power of tarot cards is it's ability to help you encode your subconscious thought patterns into a tangible image you can see and thus quickly figure things out just by looking at a series of random images on cards you draw!