After 20 years I bumped into my old Pokémon cards. I would like to know how to evaluate them and to know what is the best way to keep them safe. I tried to Google little about it but in Croatia there aren't many places to so that . Got any advices ?
The best thing you can do it get them into appropriate sleeves to protect them from UV damage / water damage / anything that might ruin the card. I highly recommend perfect fit sleeves and then double sleeving with Dragon Shield sleeves. Next, it depends on how you want to store your cards. Some people use hard cases and place them in a box or a tin, depending on how many cards you've got. I personally prefer to keep my collection in a binder since I like to organize them in specific ways (plus it's just too many to store in any kind of container). Lastly, store your collection in a cool, dry area of your house as to avoid humidity curling and other environmental affects.
As for evaluating the price, you can start by looking at the condition of each card. Are there scratches? Folds on the corners? Nicks and bumps? All of these things would grade the card as "played" condition. If you want to being appraising your collection, I recommend sourcing prices from multiple websites. Amazon, Ebay, collectors.com, trollandtoad, etc, etc. You can usually find a general idea of what the card is worth that way. Finally, if you find a true gem in your collection, consider getting it officially graded by PSA. This will exponentially increase the value of the card. :)
I hope this answered your questions as bit! Best of luck!