There are a few basics which you have to know before starting. You would have learned these simple facts in Science lessons in secondary school, however, you probably wouldn't have known that using that knowledge you could easily produce alcohol.
All alcohol starts out as starch
Enzymes are added to turn the starch into sugar with the help of nutrients
These 2 steps can be skipped by purchasing brewing sugar
Sugar is completely dissolved into water
Yeast is added to the syrup/wash
Yeast consumes the sugar and produces alcohol, creating CO2 as a biproduct
When fermentation has completely finished, the yeast will have consumed all of the sugar so you will be left with yeast, alcohol and water left in your vessel. The CO2 will eventually disperse and the yeast (poisoned by the amount of alcohol) should go dormant and sink to the bottom of the vessel leaving a mixture of alcohol and water which you can rack off (syphon) into another vessel as a consumable drink.
All shop bought vodka is is a mixture of pure alcohol and water, as above. 37.5% ABV vodka means 37.5% pure alcohol, the rest of the bottle should be water unless it is flavoured vodka.
If you would prefer a stronger ABV than you have managed to create during fermentation (usually maximum 23%), you need to distil your alcohol product. This process boils the alcohol (at around 80°c) turning it into a gas. It then collects that gas and condenses it collecting pure alcohol (up to 95% ABV) before the water boils at 100°c leaving the water behind along with any other impurities. You then need to calculate how much water to add to get the drink back to the desired strength.