Ela quer usar uma camisa branca e uma saia preta na festa.

Because Portuguese normally uses querer + infinitive directly.

So:

• ela quer usar
• ele quer comer
• nós queremos sair

English needs to after want, but Portuguese does not.

Because camisa and saia are grammatically feminine nouns, so they take the feminine singular article uma.

A very important point: grammatical gender is not the same thing as real-life gender.
So even if a camisa is a man’s shirt, the word camisa is still feminine, and you still say uma camisa.

In Portuguese, adjectives usually agree with the noun in gender and number.

Here:

• camisa is feminine singular, so branca
• saia is feminine singular, so preta

Compare:

• um sapato preto — masculine singular
• uma camisa branca — feminine singular
• camisas brancas — feminine plural

Because that is the normal word order in Portuguese for most descriptive adjectives, especially colors.

So Portuguese usually says:

• camisa branca
• saia preta
• carro vermelho

Putting the color before the noun is not the normal choice here and would sound unusual or literary.

Not exactly.

In Brazilian Portuguese:

• camisa often means shirt and can also mean a blouse or button-up shirt depending on context
• camiseta usually means T-shirt

So in this sentence, camisa is more like shirt/blouse, not specifically a T-shirt.

Na is a contraction of em + a.

• em = in / on / at
• a = the, feminine singular article

So:

• em + a festa becomes na festa

This kind of contraction is very common in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

Because Portuguese normally repeats the article when talking about two separate items:

• uma camisa branca e uma saia preta

That sounds natural and complete.
If you leave out the second uma, the sentence sounds less standard in this context.

So for learners, the safest pattern is:

• um/uma + noun + e + um/uma + noun

Yes, you could, but the feel is slightly different.

• usar is very common for wear in the sense of choosing or having on clothes
• vestir can also mean wear, but it can sometimes feel a bit more like put on / be dressed in

So:

• Ela quer usar uma camisa branca... sounds very natural
• Ela quer vestir uma camisa branca... is also possible

In everyday Brazilian Portuguese, usar is extremely common when talking about clothes someone wants to wear.

It can do both, depending on context.

Ela quer usar... can mean:

• she wants to wear it
• she intends to wear it
• she is planning to wear it

So quer does not always express a strong emotional desire. Sometimes it is just a simple plan or preference.

Yes, sometimes.

Portuguese can omit the subject when it is clear from context:

• Quer usar uma camisa branca e uma saia preta na festa.

But in Brazilian Portuguese, subject pronouns are used more often than in some other varieties of Portuguese, so Ela quer... sounds completely normal and very natural.

A rough pronunciation is:

EH-lah kehr oo-ZAR OO-mah kah-MEE-zah BRAN-kah ee OO-mah SAI-ah PREH-tah nah FEHS-tah

A few helpful notes:

• ela = EH-lah
• quer = roughly kehr
• usar = oo-ZAR
• camisa = kah-MEE-zah
• branca = BRAN-kah
• saia = SAI-ah
• preta = PREH-tah
• festa = FEHS-tah

Pronunciation varies a bit by region, but this is a good Brazilian Portuguese approximation for learners.