How do I find it ? I am new to this
Hi there! The first thing you should do is organize the cards if they're not already. The little symbol on the right side of the card (near the bottom corner of the image) is associated with the set. Gather each card from each set and arrange them from lowest number (Example: 1/100) to highest. You can find the number of the card at the very bottom, to the right, next to the rarity mark. Lastly, sort them by type (trainer / energy / etc).
For reference:
The next thing you can do is post a picture of each card here, front and back, and give us an approximation of condition. Then, we can give you a price estimate of each! You can also check sites like pokemonprices.com or get a reasonable estimation from places like Ebay, Amazon, or other online retailers.
One option some people go with is to have their cards professionally graded by PSA. This will exponentially increase the price of the card if it's rated well.. but you'll have to send it away and wait for them to return it. I also don't know if they're even still accepting cards because of covid. But it's something to consider.
Usually people sell their cards on Ebay, but you can always sell to large online retailers (trollandtoad / cardmarket) for less money. Another option is to sell to your local games shop! You'll have to call and ask if they're willing to take your stock but at least you'll be able to haggle prices and not worry about an online transaction.
I hope this helps! :) Good luck!