Para proteger o ambiente, precisamos de reduzir a poluição do ar.

In European Portuguese, precisar almost always requires the preposition de when followed by a noun or infinitive. So you say precisar de algo or precisar de fazer algo. In Brazilian Portuguese you’ll also hear precisar de fazer, though in some informal varieties speakers may drop the de before an infinitive (especially in speech), but that is non-standard.

Para + infinitive introduces a purpose clause (“in order to …”) and is very common in Portuguese. You cannot say para que + infinitive; para que must be followed by a finite verb and the subjunctive:
• Para proteger o ambiente, precisamos de reduzir a poluição do ar.
• Para que protejamos o ambiente, precisamos de reduzir a poluição do ar.

They both mean “in order that …,” but the first is more direct and uses no subjunctive.

Portuguese normally uses definite articles before abstract or generic nouns, even in a general statement. Omitting the article can make the statement sound very clipped or poetic. So you almost always say proteger o ambiente, reduzir a poluição, amar a vida, etc., unless you have a very specific stylistic reason to drop the article.

• do is simply the contraction of de + o, because ar is masculine and normally takes o as its article.
• If you were talking about pollution of “water” (a água), you’d get poluição da água (de + a).
• If you omitted the article altogether (poluição de agua), it would sound ungrammatical in standard Portuguese.

Meio ambiente literally means “environment” and is entirely correct. In everyday Portuguese, speakers often shorten it to ambiente when context makes it clear. Both are fine, but ambiente by itself is more concise.

Yes. Ter de + infinitive is another way to express obligation or necessity, and it works perfectly here:
• Para proteger o ambiente, precisamos de reduzir a poluição do ar.
• Para proteger o ambiente, temos de reduzir a poluição do ar.
They are interchangeable, though precisar de carries a slightly softer nuance (“we need to”) compared to ter de (“we have to”).

Reduzir is a regular -ir verb in Portuguese, but it undergoes a stem vowel change in the first-person singular present:
• eu reduzo
• tu reduces
• ele/ela reduz
• nós reduzimos
• vós reduzis
• eles/elas reduzem

After precisamos de, we use the infinitive reduzir (unchanged).