Give Terumah to Hashem
Give Terumah to a Kohen
שאני מתנות כהונה דכי קא זכו משלחן גבוה קא זכו
Priestly gifts are altogether different, as those who are entitled to them enjoy that privilege as guests at the divine table.
(א) האוכל תרומה שוגג, משלם קרן וחמש. אחד האוכל ואחד השותה ואחד הסך אחד תרומה טהורה ואחד תרומה טמאה, משלם חמשה וחמש חמשה. אינו משלם תרומה, אלא חלין מתקנים, והם נעשין תרומה, והתשלומין תרומה. אם רצה הכהן למחול, אינו מוחל.
(1) One who eats Terumah unintentionally must repay its principal value and a fifth whether he eats it or drinks it, or anoints himself with it, or whether the Terumah is pure or the Terumah is impure, he must pay its fifth and a fifth of that fifth [if he ate the fifth]. He may not repay with Terumah, but rather with tithed [already separated Terumah and Maaser] Chulin [non-sacred produce], which becomes Terumah. And the Tashlumin [payments for what was wrongfully eaten] also become Terumah and if the priest wishes to forego [the payments], he cannot forego.
Rules if someone eats Terumah by mistake
Pasuk stating that eating Terumah is like an "avodah"
Trumah has no designated quantity, and according to Torah law, even a single grain of wheat exempts the entire silo (Kiddushin 58b). And even though the Sages gave a required amount, this amount must not be precisely measured; so even according to Rabbinic law there is no exact amount (Rambam Trumot 3:4).
A Levi must also give maaser on his own produce, but the Rambam writes that this is only for a specific reason: because he has to give trumat maaser, which is itself a kind of trumah, on the maaser (Rambam Maaser 1:3). The implication is that were it not for the trumat maaser, there would be no need for the Levi to give maaser (Drisha Yoreh Deah 331:20).
Jewish agriculture followed a seven year cycle -- with the farming observing the Sabbatical every seventh year. On the 1st, 2nd, 4th, and 5th year of this cycle, after separating the First Maaser, a second tithe was taken from the remaining crop. This produce was taken to Jerusalem and eaten there.
Mitzvat Asseh #127, #128, #130
Maaser Rishon, Sheni, and Ani