How did they choose to elect the first president?

I am not sure what you want to know. After the colonists won the Revolutionary War, a convention was held to draw up the US constitution. After nine colonies ratified it, it became the supreme law of the land. The Constitution specifies that a President will be elected and gives details on how it should be done. Accordingly, a election was held and George Washington,who had led the colonies to victory, was the unanimous choice of the electors.