Hoy estoy enfermo, así que quiero descansar en casa.

Because Spanish normally uses estar for a temporary state or condition, and being sick is usually treated that way.

• estoy enfermo = I am sick / I’m feeling ill
• soy enfermo would sound like sickness is part of your identity, which is not what you want here

A good rule of thumb:

• ser = identity, origin, characteristics
• estar = states, conditions, location

So Hoy estoy enfermo is the natural choice.

No. In Spanish, subject pronouns are often omitted because the verb already shows who the subject is.

• estoy = I am
• quiero = I want

So Hoy estoy enfermo, así que quiero descansar en casa is completely natural.

You could add yo for emphasis or contrast:

• Yo estoy enfermo, pero él no.
  I’m sick, but he isn’t.

The adjective must agree with the gender of the speaker.

• A man would usually say enfermo
• A woman would usually say enferma

So:

• Hoy estoy enfermo... = said by a male speaker
• Hoy estoy enferma... = said by a female speaker

Así que means so, therefore, or and so. It links a cause to a result.

In this sentence:

• Hoy estoy enfermo = Today I’m sick
• así que quiero descansar en casa = so I want to rest at home

It is a very common and natural way to connect the two ideas.

Not in the same way. Porque means because, so it introduces the reason, not the result.

Compare:

• Hoy estoy enfermo, así que quiero descansar en casa.
  Today I’m sick, so I want to rest at home.

• Quiero descansar en casa porque hoy estoy enfermo.
  I want to rest at home because today I’m sick.

Both are correct, but the structure and emphasis are different.

Because en casa means at home, while a casa means to home / homeward after a verb of movement.

Here, descansar happens at a place, so you use en:

• descansar en casa = to rest at home

Use a casa with movement:

• Voy a casa. = I’m going home.
• Vuelvo a casa. = I’m returning home.

So:

• en casa = location
• a casa = direction

En casa is the normal idiomatic way to say at home.

• en casa = at home
• en la casa = in the house / in the home, often referring to the building itself or a specific house

So if you mean your usual idea of at home, Spanish often drops the article:

• Estoy en casa. = I’m at home.

Using en la casa is possible, but it usually sounds more specific or more literal.

Yes. Spanish word order is more flexible than English word order.

These are all possible, with slightly different emphasis:

• Hoy estoy enfermo, así que quiero descansar en casa.
• Estoy enfermo hoy, así que quiero descansar en casa.
• Hoy quiero descansar en casa porque estoy enfermo.

Putting hoy at the beginning is very common because it sets the time frame right away.

Yes, it is standard and helpful here. The comma separates the two linked ideas:

• Hoy estoy enfermo
• así que quiero descansar en casa

In writing, that comma makes the sentence easier to read and sounds natural. In speech, there is usually a small pause there too.

Descansar means to rest.
Dormir means to sleep.

So:

• Quiero descansar = I want to rest
• Quiero dormir = I want to sleep

You can rest without actually sleeping, so descansar is broader and fits this sentence well.

In Spain, yes — estoy malo is a very common colloquial way to say I’m ill / I’m not feeling well.

However:

• estoy enfermo sounds a bit clearer and more neutral
• estoy malo is more informal

Also, outside Spain, malo more often just means bad, so enfermo/enferma is usually safer for learners.

Hoy is pronounced roughly like oy in boy.

So:

• hoy ≈ oy

A few quick pronunciation notes for the whole sentence:

• hoy = the h is silent
• estoy = ends with the same -oy sound
• así = stress on the last syllable: a-SÍ
• quiero = roughly KYE-ro

The accent mark shows where the stress goes:

• así is stressed on the final syllable: a-SÍ

Accent marks in Spanish are very important because they show pronunciation and sometimes distinguish words from others that are spelled similarly.

So the accent in así helps you pronounce it correctly.