Learn Croatian: Još nisam umoran.

The verb biti (to be) is a bit special. For most verbs, you just put the word ne (not) in front to make them negative. But with the present tense of biti, the negative word and the verb merge into a single word. So, ne + sam becomes nisam.

Where is the word for "I" (ja) in this sentence?

In Croatian, you usually drop subject pronouns like ja (I) or ti (you) because the verb itself tells you who is doing the action. Nisam already means "I am not", so you don't need to add ja unless you want to emphasize it.

If I am a female speaker, do I say this sentence exactly the same way?

Not quite! Umoran is the masculine form of the adjective "tired", so a man would say this. If you are a woman, you need to use the feminine form. A woman would say: Još nisam umorna.

Does još always mean "yet"?

No, it often translates to "still" as well! The meaning usually depends on whether the sentence is negative or positive. In a negative sentence like this one with nisam, još means "yet" (I am not tired yet). In a positive sentence, it usually means "still" (I am still tired).