Learn Russian: Надень шлем, это опасное место. - Put on the helmet, this is a dangerous place.

In Russian, when you tell someone to successfully complete a specific, one-time action (like putting on a helmet right now), you use the perfective aspect for the affirmative command. If you were telling them NOT to do it (a negative prohibition), you would switch to the imperfective: не надевай (don't put on).

I always confuse the verbs for dressing. Why is it надень and not одень?

Use надеть (imperative надень) when putting an item of clothing or gear on yourself or on someone else. Use одеть (imperative одень) when dressing a person or a doll. A famous trick Russian school children learn is: Надеть одежду, одеть Надежду (Put on clothing, dress Nadezhda). Since a helmet is an item you put on, надень is correct here.

Шлем is the direct object being put on. Why hasn't its ending changed to show the accusative case?

It actually is in the accusative case! Because шлем (helmet) is a masculine, inanimate noun (a non-living object), its accusative form looks exactly identical to its dictionary (nominative) form.

Why do we use the neuter pronoun это if шлем (helmet) is masculine?