Chemical reactions involve changes - changes of reactants into products. Basic chemical reactions can be grouped into categories based on the types of changes that are occuring during the reaction. There are five basic categories - synthesis, decomposition, combustion, single replacement, and double replacement. If you know the type of reaction that reactants will involve themselves in, then you can predict the products that will be produced as the result of that reaction.
 

In a decomposition reaction, a relative large, complex (more atoms) compound is changed into two or more products that are smaller and less complex (less atoms). In this decomposition reaction, a compound is changed into two products that are elements.  

To correctly identify the formula of each product you need to know that there are seven elements that exist in diatomic form when they are not combined with other elements in a compound or not present in an aqueous solution. Those seven elements are hydrogen (H), oxygen (O), nitrogen (N), chlorine (Cl), bromine (Br), idoine (I), and fluorine (F). To assist in recalling this list of 7 elements, you might remember the name HONClBrIF (pronounced "Honklebriff").