Eu preciso confirmar o endereço na internet antes de sair.

Yes, you can often omit the subject pronoun in Portuguese because the verb ending already shows who the subject is.

• (Eu) preciso confirmar o endereço... both are natural.
  Including eu can add a bit of emphasis or clarity (for example, in contrast with someone else).

Both can mean I need to / I have to, but there’s a nuance:

• Eu preciso + infinitive often sounds like a personal need/requirement: I need to confirm...
• Eu tenho que + infinitive can sound more like obligation/necessity, sometimes stronger: I have to confirm...
  In everyday Brazilian Portuguese, both are common and often interchangeable.

Because precisar is followed by an infinitive to express need to do something:

• Eu preciso confirmar... = I need to confirm...
  If you used confirmo, it would become two separate ideas (and would usually need restructuring), e.g. Eu confirmo... = I confirm...

Endereço can mean either, depending on context:

• physical address: endereço (de casa / do lugar)
• web address/URL: often clarified as endereço do site, endereço eletrônico, or simply o link
  In this sentence, na internet strongly suggests you mean checking the address online (likely the location’s address, though it could also be a URL in some contexts).

Brazilian Portuguese commonly uses articles before nouns in general statements:

• confirmar o endereço is the most natural in most contexts.
  You can say confirmar endereço, but it sounds more like a sign/instruction style or more generic/less specific. In normal conversation, o endereço is preferred.

Na is a contraction of em + a:

• em = in/on
• a = the (feminine singular)
  So na internet literally = on the internet / online.
  Portuguese treats internet as feminine here: a internet → na internet.

Yes, but the meaning shifts slightly:

• na internet = on the internet / online (location/medium)
• pela internet = via the internet (method/channel)
• no Google = on Google (more specific platform)
  Examples:
• confirmar o endereço na internet (online in general)
• confirmar o endereço no Google (specifically using Google)

Both exist:

• antes de sair = before leaving (general, subject understood as the same person)
• antes de eu sair = before I leave (explicit subject, can add emphasis/clarity)
  If the subject could be someone else, Portuguese often clarifies: antes de ele/ela sair.

Sair means to leave / to go out, and context decides:

• leaving home to go somewhere: antes de sair (de casa)
• leaving a place/event: antes de sair (do trabalho / da festa)
  If you want to be explicit, you can add the place: antes de sair de casa.

Your sentence is already natural. A few common casual variations:

• Preciso confirmar o endereço na internet antes de sair. (drops eu)
• Preciso checar o endereço online antes de sair. (checar and online are very common)
• Tenho que ver o endereço na internet antes de sair. (more “I have to”)