The best way to learn poker is to learn what the hands are. All other poker games are variations using the same cards and hands.
Poker hands generally consists of the best FIVE cards.
From those five cards, the hands are ranked like this:
(lowest to highest)
High card
One pair
Two pair
3 of a kind
straight (5 cards in a row, not suited)
Flush (5 cards of the same suit)
Full house (3 of a kind + 2 of a kind)
Four of a kind
Straight flush (5 cards in a row, all the same suit)
There are 52 cards in four "suits" -- Spades, Hearts, Diamonds and Clubs. There are 13 cards of each suit. Numbered cards go 2 through 10. After that, Jack, Queen, King, Ace in order.
Cards start and end with the Ace (can be used as a one and is higher than a King). In other words, you can have a straight that goes Ace, 2, 3, 4, 5; or a straight that goes 10, J, Q, K, A.
Once you know the cards and the hands, it's just a matter of learning the rules of different types of poker games.
For example, Texas hold em is the most popular game. You get two cards only you can see and five "community" cards everyone can use. You mix one, two or none of your cards to make the best hand with betting after you get the initial two cards, betting after everyone sees the first three cards and betting on each of the next two cards.