i'm trying to determine the freezing point of a salt and water mixture. I'm confused about what i need to do. Could someone help me out with the formula?

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If we start with the water, we know the freezing point of pure water under standard conditions to be $0 ^o C $.  When we add a salt, we have freezing point depression occurring.  This is a colligative property based on molal of solute added, number of solute particles created by dissociation, and a special molal freezing point depression constant (which is often provided).

The formula:
   $ \Delta T = K_f m \times i$
Where $\Delta T$  is the change in temperature (in this case, from $0 ^o C$),  $K_f$ is the constant (in this case, for water it is 1.86), $m$ is the concentration of solute per solvent (molality), and $i$ is called the "van't Hoff factor" for how many solute particles are created.

E.g.   NaCl   -->   Na+,  Cl-   ::  2 solute particles
          CaCl2 -->  Ca2+, 2 Cl-   ::  3 solute particles