The decision on whether his side will field or bat first is taken by the captain (or someone standing in for a captain) who guesses correctly at the outcome of a single toss of a coin.

The toss must take place on the playing area between thirty and fifteen minutes before the time scheduled for the start of play.
The convention is that the captain of the home team will toss the coin and the visiting captain will usually call 'heads' or 'tails'. If he wins, he chooses whether to bat or bowl first. If he loses, the home captain makes the choice. A large number of factors will influence the winner's final decision - the state of the pitch or the light, the outlook for the weather, his knowledge of the strengths and weaknesses both of his own and the opposition's team, and so on. Far from being just a simple device for establishing who does what first, the result of the toss can sometimes be a key factor in determining the outcome of a match.
Among many improbable stories about Dr W.G.Grace, who was known for 'taking full advantage' of any opportunity that came his way, is one about his calling in a toss-up. The other captain was using a Victorian penny, with a picture of the Queen on one side and Britannia on the other. Instead of the usual 'heads' or 'tails', Grace is alleged to have shouted out - 'Woman!'