If 3 or four players, each accepts seven cards.In the case of two players then each player receives ten cards.
The dealer after shuffling the cards deals the cards to the players in the following manner Whoever picks the highest card becomes the dealer. So to choose one, spread the cards on the table and let each of the two players pick a card.
You need a dealer for the card game to begin.
Although there are many variations on how to play the game, here is the basic concept of the gin rummy game Choosing a dealer
