Learn Croatian: Prvo sam radio.

In Croatian, short helper verbs like sam are called clitics. They have a strict rule: they almost always want to be in the second position in a sentence. Because Prvo is the first word, sam must go right after it, even if it means separating from the main verb radio.

Where is the word for "I" in this sentence?

The word for "I" (ja) is left out because the helper verb sam already tells us the subject is "I". In the Croatian past tense, you usually drop the subject pronoun unless you want to emphasize who did the action.

Does the word radio tell us anything about the speaker?

Yes, it tells us the speaker is male. In the Croatian past tense, the main action word changes based on gender. Because it ends in -o (radio), we know a man is speaking. If a woman were speaking, she would use the feminine ending and say Prvo sam radila.

Can I say Ja sam prvo radio instead?

Yes, but adding ja puts extra focus on "I", sounding a bit like "As for me, first I worked." Notice that the helper word sam still stays in the second position, coming right after the new first word Ja.