I don't understand tithing. Who decided we had to give 10% every week or two?

Israelites, under the Law of Moses, were required to give 10% of their profits. The word "tithe" means to give 10%. "You shall truly tithe all the increase of your grain that the field produces year by year. And you shall eat before the LORD your God, in the place where He chooses to make His name abide, the tithe of your grain and your new wine and your oil, of the firstborn of your herds and your flocks, that you may learn to fear the LORD your God always" (Deuteronomy 14:22). Notice, though, that it was ten percent of what was gained that year, not ten percent of what they own. The funds collected were used to run the temple and pay the Levites. "Speak thus to the Levites, and say to them: 'When you take from the children of Israel the tithes which I have given you from them as your inheritance, then you shall offer up a heave offering of it to the LORD, a tenth of the tithe" (Numbers 18:26). Every third year, the money was set aside to care for the poor, as well as the Levites. "When you have finished laying aside all the tithe of your increase in the third year-the year of tithing-and have given it to the Levite, the stranger, the fatherless, and the widow, so that they may eat within your gates and be filled" (Deuteronomy 26:12).

This did not mean that this was all the Israelites gave. There were provisions in the Old Law for many voluntary offerings to God. "But you shall seek the place where the LORD your God chooses, out of all your tribes, to put His name for His dwelling place; and there you shall go. There you shall take your burnt offerings, your sacrifices, your tithes, the heave offerings of your hand, your vowed offerings, your freewill offerings, and the firstborn of your herds and flocks" (Deuteronomy 12:5-6). In a sense, the tithe was a fixed tax that supported both the Levites and Israel's welfare system.

Under the New Covenant, God did not place a fix amount to be given. "On the first day of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come" (I Corinthians 16:2). Like the Old Law, it is to be based on the amount a person propers, but the amount to give is not set. "For if there is first a willing mind, it is accepted according to what one has, and not according to what he does not have" (II Corinthians 8:12). Though an amount is not required, God does require forethought to your giving. "So let each one give as he purposes in his heart, not grudgingly or of necessity; for God loves a cheerful giver" (II Corinthians 9:7). In other words, you don't wait until the collection plate is being passed to open your wallet and say, "Well, I have twelve dollars here, I guess I'll give God two this week." You decide in advance what you want to give to God and then give as your prosperity allows.

As far as the timing goes, it was God through Paul how established that giving is done on a weekly basis (on the first day of the week, I Corinthians 16:2).

See also:

Questions and Answers regarding Giving