With a plethora of grinders available, knowing where to begin can be a little tricky.
Generally the more uniformity in ground particle size, the more even the extract can be and therefore the more clarity there will be available in the final cup.
Flat burr grinders and conical burr grinders will always give more consistency in particle size than a blade grinder due to the nature of the mechanics in the different systems. And the more fine tuning available in the grinder, the more precision one can use to extract certain characteristics of the coffee.
In its most basic form, the finer the grind size, the more extraction possible, but there is a sweet spot! The best way to find this is to brew and to taste the coffee.
If the coffee tastes dry, flat or astringent the coffee is most likely over extracted and the grind size should be made coarser. If it tastes sour or tart the coffee is under extracted and therefore you can make it finer.
Different brew methods call for different grind sizes due to of factors that help extract flavours from coffee such as pressure, or the flow rate of the water, time and temperature.
On an arbitrary scale of 1 to 10, where 1 is finest dust like consistency and 10 is very coarse grit, the brew methods would sit in an order like this:
2. Commercial Espresso
3. Home Espresso/Moka Pot
5. Cupping (a method for taste testing coffee within the industry)
6. Pour Over/Drip
7. Chemex/French Press
8. Batch Brew
10. Cold Brew
This is a very loose guide line as a starting point. As with all things coffee the proof is always in the final cup, so taste it and adjust settings to suit your preference. And above all, have fun.