How do you use selflessness in a sentence?

Since it is an adjective, you can use it either in front of a noun "the selfless firefighter" or after a form of 'to be' - is / was / will be .

The selfless firefighter risked his life to save others.

The firefighter was selfless because he risked his life to save others.

The firefighter, known for his selfless acts of bravery, was honoured.

The selfless firefighter, who was selfless, was honoured for his selfless acts, which were selfless.

As to when to use it, you'll need to understand its meaning/definition so as to know when it is appropriate. Something that is selfish, like taking your little brother's toy, is not selfless.

Hope this helps

-K