De televisie gaat uit als ik op de knop druk.

Because uitgaan is a separable verb in Dutch.

• The full verb is uitgaan.
• In a main clause, the conjugated part goes to the usual verb position, and the separable prefix moves to the end.

So:

• infinitive: uitgaan
• present tense: de televisie gaat uit

This is very common in Dutch:

• aankomen → De trein komt aan
• opstaan → Ik sta op

They are related, but not the same.

• uitgaan = to go off, to turn off
• uitzetten = to turn something off

So:

• De televisie gaat uit focuses on what happens to the TV.
• Ik zet de televisie uit focuses on the person doing the action.

In your sentence, Dutch uses gaat uit because it describes the result: the television turns off when the button is pressed.

Als is very often used for when in situations like this, especially when the meaning is close to if/whenever.

In this sentence, als ik op de knop druk means something like:

• when I press the button
• if I press the button
• whenever I press the button

Wanneer is also possible in some contexts, but als is more natural for a condition or repeated cause-and-effect situation like this.

Because als introduces a subordinate clause, and in Dutch subordinate clauses the conjugated verb usually goes to the end.

So:

• main clause: De televisie gaat uit
• subordinate clause: als ik op de knop druk

This is a very important Dutch word-order rule:

• Ik weet dat hij komt
• Als het regent, blijf ik thuis

Because with ik, regular Dutch present-tense verbs usually use the stem with no extra -t.

The verb is drukken.
The stem is druk.

So:

• ik druk
• jij drukt
• hij drukt

That is different from English, where I press and he presses only differ in the third person. Dutch also changes the form, but in its own way.

Because drukken op is the normal Dutch expression for pressing a button.

Dutch often uses a preposition where English does not.
So although English says press the button, Dutch usually says drukken op de knop.

Other examples:

• Ik druk op de bel
• Druk niet op deze toets

So op is not optional here in normal usage.

Because televisie is a de-word in Dutch.

Dutch nouns take either de or het, and this has to be learned with each noun. There is not always a simple reason that helps from English.

So the correct form is:

• de televisie

You will also often hear:

• de tv

Yes, absolutely. That is a very natural alternative.

If the als clause comes first, the main clause follows it. In that case, Dutch keeps the conjugated verb in second position, so you get:

• Als ik op de knop druk, gaat de televisie uit.

Notice that gaat comes before de televisie. That inversion is required after a fronted clause.

It can suggest a general rule or a regular result.

So this sentence most naturally sounds like:

• whenever I press the button, the television turns off

Dutch present tense is often used for:

• habits
• general truths
• instructions
• near-future events

Without more context, this sentence sounds like a normal cause-and-effect statement, not necessarily something happening only once right now.