Learn Hardcore Greek: Η αμοιβή πληρώνεται αύριο.

η is the definite article meaning the.
Here it is the feminine singular nominative form, because it goes with αμοιβή, which is a feminine singular noun.

So:

η = the
η αμοιβή = the payment / the fee / the compensation

Why is αμοιβή feminine? Does the ending -ή always mean that?

αμοιβή is simply a noun whose grammatical gender is feminine. In Greek, nouns have gender, and the article has to match that gender.

The ending -ή often belongs to feminine nouns, so it is a useful clue, but it is not a universal rule for every word in the language. The safest habit is to learn a noun together with its article:

η αμοιβή
η πόλη
η ζωή

That way you learn both the word and its gender at the same time.

What form is πληρώνεται?