How do professional sports teams choose player numbers?

I'm very curious on this. How do they choose a number to be designated to a certain player. Does a player choose the number him/herself or is it already chosen for them when they enter the league? Does it vary depending on what sport it is?How do professional sports teams choose player numbers?
In football, a player in a position can choose from a set of numbers designated to their position.


All sports: Rookies sometimes don't get the numbers they want because they are already taken by someone else on the team, and some teams retire the numbers of great players and nobody can ever wear the number on that team.How do professional sports teams choose player numbers?
Well for hockey you choose it yourself. Sidney Crosby who is going to be one of the best players in the NHL chose number 87 for himself because he was born on August 7th 1987. Thats 8.7.87
The players get to choose their own numbers. Each player has their own reason for choosing their number, some feel it is good luck, some just for random reasons.