If you decide to get serious about it, then try to learn as much about the cards as you can. There are magazines about cards. Also ask a lot of questions and talk to other collectors. You have to figure out how much money you are willing to spend and then do not spend more than your budget. You can shop around and find the best price for the card you want in your collection.
Some people pay a lot of attention to the prices in price guide books.
These books list how much each card is supposed to be worth, but a lot of times these price lists are too high. More often than not, you will be able to get a card for less. Try to beware because there are counterfeits out there. It is illegal to print counterfeit cards. They are usually printed on cardboard that is thicker or thinner than the real cards, and often have blurry photos.