What exactly, is "Reishis Degancha ... "?
Rashi: "Reishis Degancha ... " refers to Terumah. 1
Rashi, Devarim 26:13): The first from the time that it became Dagan - following the Miru'ach (flattening the pile that follows the winnowing). See also Ba'al ha'Turim and refer to Bamidbar, 18:12:1:1*). We also learn from here that Terumah only falls due after the crops have been detached - Rashi (citing Kidushin, 62a).
What exactly, is "Reishis Geiz Tzoncha"?
Rashi #1: It is the first of the annual crop of wool 1 of all one's sheep.
How much is one obligated to give the Kohen of one's corn, wine and oil and of one's sheep's wool?
Rashi (citing T'rumos, 4:3): Although the Torah does not specify the amount that one is obligated to give, the Chachamim gave the Shi'ur for Terumah 1 as a fortieth - for a generous person; a fiftieth - for an average person, and a sixtieth 2 - for a person who is miserly; and the Shi'ur for Reishis ha'Gez as a sixtieth, which one must give from a minimum of five sheep. 3
Targum Yonasan: The minimum Shi'ur of Reishis ha'Gez is enough wool to manufacture a belt for the Bigdei Kehunah. 4