You will need to determine the concentration of the acid (it usually says on the bottle, or you can work it out from the information on the bottle) and then perform a standard dilution calculation to work out how much acid you need.
C1.V1 = C2.V2
Where: C1 = concentration of solution 1; V1 = volume of solution 1; C2 = concentration of solution 2; V2 = volume of solution 2.
Remember that C1 and C2 must be in the same units, and volumes V1 and V2 must be in the same units.
And this bit is very important:
Make sure you are wearing the correct safety gear (e.g. lab coat, safety specs (full face shield would be good), and gloves, and add the acid SLOWLY to the water.
Always add the acid to the water.
NEVER add water to the acid as this many cause spitting and the last thing you want is concentrated
Also, keep an eye on the temperature of the solution as it will get hot. Allow it to cool if it becomes to warm.
Bottom line: BE CAREFUL
(I personally hate diluting acids down, and I have been doing it for years...)
If you post information on the concentration of the stock acid I will walk you through the calculations.