How do you balance the equation mg + hcl?

You cannot balance this, as you do not have any products, only reactants. If you add the correct products like this, you get

Mg + HCl → MgCl₂ + H₂

Therefore, we can balance it like this...

Mg + HCl → MgCl₂ + H₂ left hand side: 1Mg; 1H; 1Cl right hand side:1Mg; 2H; 2Cl

Start with the most complicated formula, MgCl₂.

Mg is balanced, so balance Cl next. Put a 2 in front of the HCl.

Mg + 2HCl → MgCl₂ + H₂ left hand side: 1Mg; 1H; 2Cl right hand side:1Mg; 2H; 2Cl

Everything is now balanced. The final balanced equation is

Mg + 2HCl → MgCl₂ + H₂