Hoje eu vou faltar ao trabalho porque estou com febre.

Yes, you can often omit eu because Portuguese verb forms usually make the subject clear.

• Hoje vou faltar ao trabalho... sounds natural and common.
  Including eu adds a bit of emphasis/contrast (like “I am going to…”).

Vou + infinitive is the very common “near future / planned future” in Brazilian Portuguese, similar to “I’m going to…”

• Hoje eu vou faltar... = “Today I’m going to miss/skip...”
  You can also use the simple future (faltarei), but it sounds more formal and less common in everyday speech.

Faltar often means “to be absent / to miss (an obligation/event).” In this sentence it means to miss work / not show up to work.
Common patterns:

• faltar a + place/event: faltar ao trabalho, faltar à aula (miss work/class)
• faltar com algo can mean “to fail to do something” or “to be lacking” depending on context.

Because faltar typically takes the preposition a: faltar a/ao/à (to be absent from).

• a + o = ao → faltar ao trabalho
  No trabalho (= “in/at the workplace”) describes location, not absence. So faltar no trabalho is generally not the standard phrasing for “miss work.”

They mean the same thing, but ao is the required contraction in standard Portuguese:

• a + o → ao
  Similarly:
• a + a → à (with the accent)
  So ao trabalho is “to the work / to work” in form, but with faltar it’s idiomatically “miss work.”

In Brazilian Portuguese, for many temporary conditions/symptoms you use estar com + noun:

• estou com febre (I have a fever)
• estou com dor de cabeça (I have a headache)
  You may also hear tenho febre, but estou com febre is extremely common for a current, temporary state.

Here porque means because (it introduces a reason).
Quick guide:

• porque = because (Estou em casa porque estou doente.)
• por que = why / for which (Por que você faltou?)
• por quê = why (at the end) (Você faltou por quê?)
• porquê = the reason (a noun) (Não sei o porquê.)

It’s flexible. Some natural variants:

• Hoje eu vou faltar ao trabalho porque estou com febre. (original)
• Eu vou faltar ao trabalho hoje porque estou com febre.
• Porque estou com febre, hoje eu vou faltar ao trabalho. (more “written”/emphatic)
  Most often, porque introduces the reason clause after the main clause, as in the original.

Common pronunciation points in Brazilian Portuguese:

• hoje: the j sounds like the s in “measure” (zh sound) → roughly OH-zhee
• eu often sounds like ew (a bit like “eh-oo” blended)
• vou: rhymes roughly with “so” (closed o)
• trabalho: the lh is like ly (million) → tra-BA-lyu
• estou often sounds like is-TOH (initial e can sound like i)
• com often sounds like kong (nasal) depending on accent
• febre: FEH-bri (final e often becomes an i-like sound)

It’s neutral and perfectly normal in everyday conversation. Common alternatives:

• More casual: Hoje vou faltar no trabalho porque tô com febre. (very common speech, though faltar ao trabalho is more “standard”)
• More explicit: Hoje não vou trabalhar porque estou com febre. (I’m not going to work today because I have a fever)
• More formal: Hoje faltarei ao trabalho, pois estou com febre. (more written/formal; pois = because)