Lets be honest, most of the time you probably don't even need to have a substrate at all however, it's said to reduce fish stress, hold beneficial bacteria, looks more aesthetically pleasing, etc (if you're asking this question than you probably already know that you yourself want to add a substrate to your aquarium).
First off their are many different variables in trying to answer this question such as... what's your substrate material (sand, stone, gravel, dirt, combination, etc)?; is there an under-gravel filter?; how many gallons is the aquarium?; what are the dimensions of the aquarium?; will you have live or fake plants?; etc. Also keep in mind that the more pounds of substrate you put into your aquarium, the less amount of water that will be going into your aquarium since your substrate is more dense than water. The less amount of water your tank has, the less fish you'll probably be able to keep.
In actuality there is no set amount of how much substrate you should have in your aquarium so, this question is really just personal preference and what you feel works best for you. A lot of people will tell you that it's 2lb of gravel per 1 gallon of water, or even 1lb of gravel per 1 gallon of water. Well that really isn't a good method to teach people because tanks have different dimensions and shapes.
So when faced with a method as to how much substrate to add to your aquarium, I'd recommend using how many inches, centimeters, etc of substrate you want. Maybe you like a 2 inch dense amount of substrate, or maybe even 1 cm, or maybe you want some areas in your aquarium to have more or less substrate. For those who are Takashi Amano enthusiasts/fans like myself I'd say it's anything from a thin layer of substrate that completely covers the base up to 2 inches although most of his tanks appear to be around an inch. I'd say for most aquariums you probably don't need anything more than 3 inches but it may also depend on the density of the substrate material you're using (like rocks, stones, etc).
As for what type of substrate is best that depends on what you're hoping to achieve in your aquarium. Most beginners usually use the popular 'pea gravel' which is just your basic gravel rocks that are about the size of a pea and you can find this in all types of colors from pink, black, red, white, blue, neon, etc. The downfall to bigger substrate materials is that it creates lots of pockets between the gaps of the substrate which allows for toxic gases and such to form under the substrate, and some types of fish could possibly get the 'pea gravel' lodged into their throat. Most people seem to think 'sand gravel' is more likely to get consumed by fish and harm them but this is not true, as fish in the wild generally live in soil substrates. Also, since the 'sand gravel' is much finer than 'pea gravel' it allows less pockets for toxic gases and such to form. 'Sand gravel' is also not as hard to vacuum as you may imagine, I find it about the same as 'pea gravel' except the 'sand gravel' doesn't get lodged into the mouth of the vacuum.