Here's an example of how to do your calculation:
You have a solution containing 5mL of an unknown concentration of H2SO4. To figure out the concentration, you add 0.1M NaOH until the solution is fully neutralized. You find that it takes 15.8mL of your NaOH solution to neutralize the H2SO4. How many moles of H2SO4 were in your unknown solution?
To find the number of moles of NaOH we used, we recall the definition of molarity. Molarity (M) = moles/L. Therefore, to find the number of moles of NaOH in 15.8mL of 0.1M solution of NaOH, we multiply the concentration by the volume:
0.1 moles/L * 0.0158L = 0.00158 moles
So, in our reaction we used 0.00158 moles of NaOH. Now, the reaction which you track is the following:
2NaOH + H2SO4 --> 2H2O + Na2SO4
From the balanced chemical reaction, we see that NaOH and H2SO4 react at a 2:1 ratio. Therefore, when the reaction is complete, the number of moles of NaOH reacted = (1/2) * the number of moles of H2SO4 reacted. So, we must have started with (1/2)* 0.00158 = 0.00079 moles of H2SO4