If you don't already know, first learn the composition of a standard deck of playing cards, the card names or terms. e.g.
A standard deck or pack of 52 playing cards in the USA is based on the four French suits; spades, hearts, diamonds, and clubs.
Each suit has 13 cards, each card having its own rank, giving each suit a total of 13 ranks; 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, King, Ace. The ace can have a rank of 1 (low) or be the highest ranking card in the deck, depending on the game and/or hand it appears in. There are many more instances where the ace is ranked high than instances where the ace is allowed to be ranked low.
A card with the rank of two (2) is also called a duece, which is sometimes affectionately referred to as 'duecey'. Why cards need cutesy nicknames, I don't know, but there are a crap load of them used in poker, so you have little choice but to learn them.
So on and so forth...
Then learn the hierarchy or ranking of valid hands in standard poker. That is, which hands out-rank others.
There are all kinds of non-standard hands for all kinds of non-standard games of poker. Don't worry about those until you have a solid grasp of standard poker, since most games of non-standard poker presume (if not require) a solid understanding of standard poker.
Once you have mastered the composition of a deck, the ranking of each card, the common nicknames, the valid hands in standard poker, their composition, nicknames, and ranking, learning the rest will be inevitable.