Geez. This isn't that complicated. First, are they good enough to sell - untouched (always kept in cases)?
If so, go on E-Gay and type in "pokemon" or "yu-gi-oh" and sort by price, starting with the most expensive. Make a list of the most valuable ones, making sure you understand about editions, quality, etc.
See if you have any of the cards that are worth bucks.
Then, sell them.
Or, you can take them to a card shop, or put an ad in the supermarket or something. Less bother. Good luck.