A cena começa agora.

A is the feminine singular definite article, so it means the.

• a cena = the scene
• uma cena = a scene

Portuguese uses articles very often, sometimes more often than English does.

Because cena is a feminine noun in Portuguese.

• a goes with feminine singular nouns
• o goes with masculine singular nouns

So:

• a cena
• not o cena

This is grammatical gender, not biological gender. You simply have to learn the noun with its article.

Cena usually means scene, especially in theater, film, TV, or a story.

Examples:

• a primeira cena do filme = the first scene of the movie
• a cena final = the final scene

It can also sometimes mean a scene in the sense of a dramatic situation or public display, depending on context, but in this sentence the most natural meaning is the regular scene of a performance or story.

Começar is the infinitive, meaning to begin / to start.

In the sentence, the verb is conjugated:

• começar = to begin
• começa = begins / starts

Since the subject is a cena (the scene), which is third-person singular, the verb also has to be third-person singular:

• A cena começa. = The scene begins.

Começa is the present tense of começar, third-person singular.

Basic present-tense forms:

• eu começo = I begin
• você / ele / ela começa = you/he/she begin(s)
• nós começamos = we begin
• vocês / eles / elas começam = you/they begin

So A cena começa agora literally matches The scene begins now or The scene is starting now, depending on context.

The ç is called c-cedilla.

It tells you that the c is pronounced like s before a, o, or u.

So:

• começa sounds like ko-MEH-sa

Without the cedilla, ca, co, and cu would normally have a k sound.

Compare:

• casa = KAH-zah
• começa = ko-MEH-sah

A natural Brazilian Portuguese pronunciation is roughly:

a SE-na ko-MEH-sa a-GO-ra

A few notes:

• cena stresses the first syllable: SE-na
• começa stresses the second syllable: ko-MEH-sa
• agora usually sounds like a-GO-ra

In connected speech, the final a of começa and the initial a of agora may flow together smoothly.

Because the subject is already stated: A cena.

Portuguese does not need an extra pronoun here. In fact, adding one would usually sound unnecessary:

• A cena começa agora. = natural
• A cena ela começa agora. = generally not the normal choice

Since a cena is the subject, the sentence is already complete.

Yes, agora can move, although the original sentence is very natural.

Possible versions:

• A cena começa agora.
• Agora a cena começa.

Both are grammatical, but the first one is the most neutral and straightforward.

Putting agora first can give a little more emphasis to now.

Usually, no. In normal Portuguese, you would normally say A cena começa agora.

Leaving out the article sounds unnatural in most standard contexts.

Portuguese often requires articles where English may not focus on them as much.

Yes. Even though the Portuguese uses the simple present (começa), in context it can often sound natural in English as either:

• The scene begins now
• The scene is starting now

Portuguese present tense is often used in ways that overlap with English simple present and present progressive, depending on context.

Yes. Depending on the situation, you might also hear:

• A cena tá começando agora.
• A cena está começando agora.

These mean The scene is starting now more explicitly.

Notes:

• está começando = is starting
• tá começando = informal spoken version of está começando

Your original sentence, A cena começa agora, is completely correct and natural, especially in a more neutral or scripted style.