Learn Italian: Il medico lavora oggi. - The doctor is working today.

Lavorare (to work) is a regular -are verb. Because the subject is il medico (the doctor, which is a "he" or "she"), we drop the -are and add the third-person singular ending, which is -a. If you were talking about yourself, you would use -o: io lavoro (I work).

The English translation says "is working". Why isn't there a word for "is" in the Italian sentence?

In Italian, the simple present tense covers both general facts ("works") and actions happening right now ("is working"). You don't need to translate "is" or add an extra ending. Lavora means both "works" and "is working" all by itself!

Is medico always masculine? What if the doctor is a woman?

The word medico is grammatically masculine, which is why it takes the masculine article il. It is very commonly used as a professional title for a doctor of any gender. If you want to specify a female doctor, you can use the word la dottoressa.

Could I put oggi at the beginning of the sentence instead?

Yes! Italian is quite flexible with words that tell you when something happens. Saying Oggi il medico lavora is perfectly natural and just puts a little more emphasis on the word "today".