Look at an example. Let's say a certain acid has a concentration of 1 mole/liter in solution. The denominator is [H2A] = 1 in this case. Furthermore, let's say that it dissociates into only [H+] and [HA-], therefore [H+]*[HA-].
At this point, our equation is: Ka1 = [H+]*[HA-]/1, or just Ka1 = [H+]*[HA-]. If H+ concentration is measured to be 0.0002 mol/liter at that point, then [H+]=[HA-]= 0.0002.
Therefore, Ka1 = (0.0002)*(0.0002)/1 = 4(10^-8).
Ka1 calculations are done in such fashion. Conceptually, they require knowing multiplying/dividing, and in case of very small numbers such as in this example, dealing with scientific notation and exponents.
Establish the precise acid for which you want to find the first dissociation constant (Ka1). Prior to calculating an acid's Ka1 value, it makes sense to experimentally verify that its acidic effects (H+, HA-, and H2A concentrations) can be measured without excruciating cost, safety risk or error. Make sure that H+ contribution for that acid is measurable and distinguishable from any other acidic compounds and the background "noise" from all instrumentation. Label that acid as the H2A compound, with H+ and HA- being the corresponding dissociated forms of that acid.
Determine the Ka1 numerator. The numerator (top part of the Ka1 ratio) contains a product of concentrations---hydrogen (H+) concentration multiplied by acid (HA-) concentration.
A note on notation: the "HA-" refers to a general molecular "core"---the "A"---and at least one other hydrogen atom. There must be one or more hydrogen atoms attached to the remaining acid. If not---if the acid is "A-"---then it is not polyprotic. Ka1 becomes meaningless since there is no Ka2 that corresponds to at least one other hydrogen that can be dissociated. This numerator is divided by a concentration of acid that is not dissociated (H2A).
Plan measurement and detection schemes. To find the first dissociation constant (Ka1), all [H+], [HA-] and [H2A] values have to be known. A good first step is to use pH at a certain point in a titration. Knowing pH is useful, since pH = -log[H+]. Therefore, having the pH value gives [H+] through the relation [H+] =10^(-pH).
Account for measurement limits. If the required concentrations are measured to, for instance, three decimal places, it makes no sense to report Ka1 calculation results to five or six decimal places. Recall that the calculation isn't "pure mathematics," and has to relate back to the real world. Therefore, significant digits and measurement error must be taken into account when reporting Ka1.