Players are wising up to the fact that some cards are expensive simply because there are so few of them in circulation, and with Masters sets being printed in such high supply, these types of "chase" cards lose a lot of their luster because the massive influx of supply means they lose much of their value
They're generally written "[Cost]: [Effect]." Some keywords are activated abilities and will have colons in their reminder text.
You can build him in a Lord of the Rings themed deck, focusing on the heroes of Middle-earth, or in a pure good stuff build, with all the best cards you can find until you out-value your opponents with your cards
In general, you should avoid sideloading sleeves
Magikarp is basically useless, doesn't deal much damage, and can be easily killed
As the commander of a Mono White deck