Hi there, I will try my best to explain it
The first step is to find the percentage of oxygen in the compound, so you have it for reference:
100% - 36.5% - 25.5% = 38%
So there is 38% oxygen in the compound.
Now we divide each percentage of each compound by its atomic mass so we can work out the ratio of each compound.
Sodium: 36.5 / 23 = 1.586956522
Sulphur: 25.5 / 32.1 = 0.794392523
Oxygen: 38 / 16 = 2.375
As you can see, the number for sulphur is the smallest (0.79...). Now to work out the ratio of each compound you divide each number shown above by the smallest number out of the three (in this case, sulphur's). I hope that makes sense!
Sodium: 1.586... / 0.794... = 1.99... -> rounds up to 2
Sulphur: 0.794... / 0.794... = 1
Oxygen: 2.375 / 0.794... = 2.98... -> rounds up to 3
So the overall ratio is Na2SO3. Empirical formula is the simplest whole number ratio of a compound, and the ratio shown cannot be simplified anymore than it already is.
I hope that is helpful, if there is something I haven't made clear feel free to ask