Both men and women are obligated in this mitzvah (Sefer Hachinuch 479). Children who have reached the age of chinuch [approximately ages 5-6] should be trained in this mitzvah (Laws of Tzedakah and Maaser pg. 32). Even a poor person is obligated to give charity (Shulchan Aruch YD 248:1). Theoretically, two poor people may fulfill their obligation by giving
charity to one another
(Orchos Rabbeinu 1:pg. 302). It is important to note, generally speaking, women are only obligated if they are financially independent. Therefore, a married woman fulfills her obligation through her husband [and, in many circumstances, should only give a substantial amount of money to tzedakah with the consent of her husband] (Laws of Tzedakah and Maaser pg. 27-32).
When the Torah describes the commandment to give
, the language used is, "A person should give the poor person
(fill what he lacks). In other words, a person is required to give another person that which he is lacking. The Sefer Hachinuch  writes this mitzvah is not limited to the rich giving to the poor. Rather it applies even in situations where a wealthy person is lacking something [e.g. he does not have access to his wallet at this particular moment] and a poor person will be able to fill that need.
Practically, the Rambam [Hil' Matnos Aniyim 7:3] explains, if someone is lacking clothing, the obligation is to get him clothing. If he is lacking household furniture, you are obligated to get him those items, and if he is lacking
a spouse, you are obligated to help him find a spouse. Additionally, if someone was accustomed to a certain financial status [e.g. a person is used to having household help], and now cannot afford this, you are obligated to provide this, since these items are currently lacking for him. It is important to note, the
write that one fulfills his obligation by giving money to the person lacking in order that they can buy the object they specifically need (The Laws of
Tzedakah and Maaser pg. 5 ftnt.7). Rambam concludes that the obligation is to take away what one is lacking, but not to make him rich. Additionally, according to the Sefer Hachinuch , included in this commandment is saying kind and comforting words to another.
One is only considered "lacking" if he was used to this status before he became poor. If however, he developed an "expensive" lifestyle after one becoming poor, you are not obligated to fulfill his newly acquired taste (Aruch Hashulchan YD 250:2).
If one is lacking funds to pay for a
], there is a dispute whether this is included in the obligation to give tzedakah. According to the Chafetz Chaim,zt"l, this is included in tzedakah
(Biur Halachah 671:1 s.v. v'afilu). Rav Moshe Feinstein, zt"l, argues that although this is a very nice thing to do, it is not considered tzedakah (Igros Moshe YD 2:141:3 s.v. u'payrish).
The Rambam [Hilchos Matnos Aniyim 7:1] and Shulchan Aruch [YD 249:1] write there are three different levels when it comes to performing tzedakah giving
1) 20%, 2) 10% or 3) less than 10%. One who gives 20% is fulfilling the mitzvah the best possible way, 10% is considered average, and one who gives less than 10% is considered stingy.
From a Biblical standpoint, as long as one has given 1/3rd of a
over the course of an entire year, he has fulfilled his obligation (Shulchan Aruch YD 249:2). In USD this is a very small amount and depending on the value of silver, this can be less than one dollar (see Laws of
Tzedakah and Maaser pg. 8). As an aside, in order to ensure that each person fulfills at least the minimum level, the custom became to collect tzedakah during davening (Shach YD 249:4).