If you titrate with sodium hydroxide solution, the reaction is:
Just as an example, lets say you had 100 ml of orange juice, which weighed 120 g. You titrated this against 0.1 M NaOH solution, and measured volume of titre as 20 ml.
First, work out how many moles of NaOH you used.
The equation tells you that 1 mole of citric acid reacts with 3 moles of NaOH, so you must have had (0.002/3) = 0.00067 moles of citric acid in your orange juice.
The molar mass of citric acid is 192.12 g, so 0.00067 moles is 192.12 x 0.00067 = 0.1287 g of citric acid.
The % of citric acid was therefore:
Please note that, with the exception of the reaction equation and the molar mass, I just made these numbers up to illustrate the procedures - I can't recall how much citric acid is really in orange juice!