Check on the bottom of the card. If you see a circle, it is common. If you see a rhombus, it is uncommon. If you see a black star, it is rare. If you see a red star, it is very rare. If you see a star that has the word "promo" across it, it is a promotional card. If it is a prerelease Raichu card or an illustrator card, keep it and preserve it at all costs. I would price them: 10 cents for regular common cards, 50 cents for holographic common cards, 1 dollar for full art common, 1.10$ for full art + holo common (very unlikely to exist), 25-50 cents for the uncommon (you be the judge), 50-65 cents for uncommon holo cards, 75-1.25 for full art uncommon, 1.30-2 dollars for full art and holo cards that are uncommon, 35-75 cents for rare cards, 0.75-1 dollar for rare holos, 0.90-1.40 for full art rares, and 1-2 dollars for full art and holo rares. Promotional cards can range in prices, and it is hard for me to tell how much each card is worth the higher up it goes. You also have to take into consideration about the date when it was printed, the expansion pack it is from, if it is counterfeit (it is illegal to sell counterfeit cards), and many more variables. Note that these are just estimates, and they are not exact or even accurate. If you can send me clear pictures of the card, I can consult a friend who can help you (he isn't on PokeBase, though). Also, if you have unused TGCO redemption cards, select 10 random Pokemon for every card from the same extension packs, and multiply their values by two.
Source: personal experience