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