Do you say first or firstly when start a sentence with it?

You can say either "first" or "firstly" to start a sentence, but there is a slight difference in usage:

* "First" is more common and generally preferred in everyday language.

* "Firstly" is a bit more formal and often used when listing several points.

Examples:

* First, let me introduce myself.

* Firstly, we need to define the problem.

* First we will go to the store, then we will go to the park.

Ultimately, both are grammatically correct and the choice depends on your desired tone and context.