The general Minhag is to give a commemoration of the Machtzis HaShekel at Mincha time on Ta’anis Esther. It is customary to give three coins, each one of which has a face value of “one-half”, it whichever currency is used in that location. This would mean that in the US one would give 3 half-dollars, in Eretz Yisroel 3 half-shekels etc. Some opinions consider it preferable to give silver coins, or to buy and return them, when available.
This money should be given specifically to the poor, and not to other causes.
The basic Halacha only requires men above 20 years old, and some say 13, to give Machatzis HaShekel. However the prevalent Minhag is to give even on behalf of young children, and even for women and girls. While one is not obligated to give on behalf of young children, once he starts to do so he must continue in subsequent years.