Use if, else if and else to run code depending on a condition.

if (health <= 0)
{
    Die();
}
else if (health < 25)
{
    Debug.Log("Low health");
}
else
{
    Debug.Log("Alive");
}

Comparison operators:

== != > < >= <=

Logical operators:

&& // AND
|| // OR
!  // NOT