As Rosh HaShana approaches, I want to make sure that I have a handle on the Halachot of how much Tzedaka (what percentage of income) one is required to give. Specifically; How much is one required to give? Is that before taxes or after taxes? Also, I am a recent Oleh to Israel, and was therefore given a significant amount of money from the Israeli government as one of my Zechuyot. Am I also required to give a portion of this amount?

There is a custom to give 10% of one's net earnings- after deducting necessary expenses: food, taxes, electricity, rent. Education of one's older children and providing them with food is considered Tzedaka. Your Zechuyot should be considered earnings but don't forget to deduct the expenses that those Zechuyot were meant to cover.