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?

πληρώνεται is the 3rd person singular present mediopassive form of πληρώνω.

In simple learner terms, it means:

it is paid
it gets paid
sometimes it is being paid, depending on context

The ending -εται is a very common mediopassive ending in Modern Greek.

Why is the verb in the present tense if the sentence says ?