Finding the theoretical yield involves using stoichiometry based on a balanced chemical equation. You would first convert the amounts of your reactants to moles, use the mole ratio from the equation to find moles of product, and then convert back to grams if needed. It's essentially predicting the maximum amount of product you could get under perfect conditions. What particular reaction are you working on? Or is this for a class you're taking? I'm here to help you through it!