What's with the Tithe?

I know my title makes it sound like I am against tithing, and that is not the case at all. I am reading a chronological Bible this year (one reading each day and the readings are done in the order they happened), and so far I am up to date with all the readings. Just the other day I read what I think is the first mention of the tithe in the Bible. In Genesis 14, there is a record of Abraham conquering several kings. Toward the end of the chapter, one king (Melchizedek), praised Abraham and at the end of verse 20, the text reads "Then Abraham gave him a tenth of everything." Is this the first mention of a tithe? If so, it doesn't seem to make much sense with how we think of a tithe today.

Several chapters later Jacob has a dream and God promises Jacob that he will be a blessed nation and that God will be with him always. Jacobs response? He promises God that he will give 10% of everything that is given to him. How does Jacob do this in the time of the Bible?

I know there are many more examples of tithing in the Bible, but these brief accounts have led me to wonder what the purpose of the tithe is, and what it should be. Is the purpose to pay the pastors of churches? If so, it must be much nicer to be a pastor of a megachurch than of a church with 20 members. Is a pastor being rewarded for having a larger congregation? Does that even make sense?

I have many more questions than answers, so maybe I will take some time to look into this matter and write more in the future. At the least, it made me want to know where my tithe is going...