You can get blades from 0.1 mm to 19 mm. Choose a blade that leaves the length that you want to have.

If you buy a new clipper they usually include a blade called nr 10 that leaves 1.5 mm. That blade is good for clipping in armpits and around the hygiene area where you want it really short. And can be good if the dog has mats behind the ears as it’s safer than a scissor there.  You can also use blade nr 10 under the attachment comb. 

The number on the blade doesn’t reflect the length they leave -so an nr 4 doesn’t leave 4 of anything. It’s just a number. All manufacturers use the same numbers for the same lengths.

You will also notice that some blades just have a number and others have a number and then an F or FC behind the number. 

The letters indicate that the blade has a full set of teeth and that gives you a smoother finish when you clip. The blades that don’t have a letter after the number is called skip tooth blades and have a wider space between the teeth.

It’s handy when you clip a matted coat as they can get into the mat- but it’s a higher risk of nicking the dog when you use them. 
So I always advise beginners to use the F blades for safety reasons. 

You can buy secondhand clipper blades in the same places as the clippers- you often find ads with both clippers and blades together. 
Or if you buy a new clipper – look if the seller has a deal on a clipper plus blade. 

Google around for prices -it can be a huge variation in the price.

This video shows you how to attach and detach the blade