We hear that the 10% comes from the word tithe in the Bible. The first place we see it is in the book of Numbers:
Numbers 18:21-26 21 To the Levites I have given every tithe in Israel for a possession in return for the service that they perform, the service in the tent of meeting. 22 From now on the Israelites shall no longer approach the tent of meeting, or else they will incur guilt and die. 23 But the Levites shall perform the service of the tent of meeting, and they shall bear responsibility for their own offenses; it shall be a perpetual statute throughout your generations. But among the Israelites they shall have no allotment, 24 because I have given to the Levites as their portion the tithe of the Israelites, which they set apart as an offering to the LORD. Therefore I have said of them that they shall have no allotment among the Israelites. 25 Then the LORD spoke to Moses, saying: 26 You shall speak to the Levites, saying: When you receive from the Israelites the tithe that I have given you from them for your portion, you shall set apart an offering from it to the LORD, a tithe of the tithe.
In this section, we see that the tithe is very specific--it is given to the Levitical priests in order that they perform their duties in the temple. Because they have these specific duties, they will not receive an allotment like the rest of the nation. And it is then a tithe of the tithe that is given to God.
In Deuteronomy 14, we see that the tithe is agricultural:
22 Set apart a tithe of all the yield of your seed that is brought in yearly from the field.
23 In the presence of the LORD your God, in the place that he will choose as a dwelling for his name, you shall eat the tithe of your grain, your wine, and your oil, as well as the firstlings of your herd and flock, so that you may learn to fear the LORD your God always.
28 Every third year you shall bring out the full tithe of your produce for that year, and store it within your towns;
It has a specific use; it is eaten at the temple.
Today, we hear that giving 10% is about giving our money, and our time, and our talents (the 3Ts: tithe, time, talents). I don't disagree that we should give these things. But if we are going to use these verses in the Tanach (Hebrew Bible; Old Testament) to make our case, why do we ignore so many other things there?
Why do we not observe so many food laws? We see in Leviticus 11 that God commands His people to not eat pig (Leviticus 11:7 The pig, for even though it has divided hoofs and is cleft-footed, it does not chew the cud; it is unclean for you. ), yet pork is one of the most highly eaten meats today, and ham has become a traditional Easter dish.
Some may say that those laws are not moral laws and therefore they do not have to be followed today. But how is the example of the tithe a moral law? Moral laws are more along the lines of having to do with not murdering or not stealing or other laws such as those.
Perhaps the biggest issue is that we simply do not study our Bibles and try to understand the context in which the text was formed. We just look for ways to apply it to ourselves, and therefore, we pick and choose what we deem applicable. If we are going to do that, we at least should be honest about it and be upfront that we don't abide by everything in there.