What most people do to organize and sell their cards is the following:
Find out what set the card is from. In older cards, the set symbol is located just below the bottom right corner of the illustration of the card. For example, a card from the Jungle set will have a symbol similar to a Vileplume's flower in said spot. In newer cards, the set symbol is located in the bottom right corner of the card itself, right beside its rarity symbol. To match up symbols with sets, go to this link: http://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_Trading_Card_Game_expansions
Check its rarity. Every card has a set rarity and a symbol to represent that, from common (a circle), uncommon (diamond), and rare (star). In the Gold/Silver sets, Shining Pokemon had three small stars in the rarity place, symbolizing it's extreme rarity. In newer sets, ultra rares and certain holos tend to have a holofoil star. The rarity symbol is located in the very bottom right corner of the card.
Is it First Edition? In the older sets, cards that were printed early got a stamp that read "1st edition" outside the bottom left corner of the illustration box. First edition cards are a lot more valuable than the unlimited versions, which are lacking the stamp. Keep an eye out for those!
Know its condition. Most people will want a card that's in very good shape, or in mint condition. If the card is bent, has edge wear, or if holofoil cards have the holo sheet peeling off, then the card will not go for much at all. Always protect your cards from further damage, if any. The condition of a card says a lot about its value. You could have a secret rare Crystal Charizard from Aquapolis, which goes for a whole lot in mint condition. But if it's bent, has edge wear and has peeling foil, it's not gonna go for more than $20-$30 or so. (I could be wrong about this estimate, don't take my word for it!)
Have them graded. If you have very good conditioned rare cards, you should mail them to companies that thoroughly examine cards, give them a grading from 1-10 (10 being gem mint, the best possible grading) for a pretty reasonable price. They then put them in a hard plastic case to protect them and prevent any damage to the card whatsoever, then mail them back to you. Graded cards actually tend to go for more than cards in a sleeve, as cards that are graded are pretty much guaranteed to stay in the best condition possible forever. Please note though, graded cards aren't usually worth much unless the grading is 7 and up!
Look for average prices online. If you want to sell a card, you should look up the average price the card you're selling goes for. For example, a first edition Base Set Charizard holo typically sells for a couple hundred dollars if in mint condition, and even $1,000+ if it's graded at a 10!)
This is the most information I can think of off the top of my head. I really hope I've helped!