Learn Hardcore Croatian: Natjecanje počinje večeras.

In Croatian, the present tense is often used for a scheduled or expected future event, just like English can say The competition starts tonight.

So Natjecanje počinje večeras is completely natural and means that the event is set to begin tonight.

A more explicitly future version is also possible:

Natjecanje će početi večeras = The competition will begin tonight

Both are correct, but the present tense version often sounds very natural for timetabled or planned events.

What is the basic form of počinje?

The basic dictionary form is početi, which means to begin / to start.

Počinje is:

3rd person singular
present tense

So it means:

it begins
it is beginning

In this sentence, the subject is natjecanje, so počinje agrees with it:

Natjecanje počinje = The competition begins

What kind of noun is natjecanje?