Just buy a collar for a puppy, breeder or pet shop will be able to advise you, you won't need to measure it.
When you put it on don't make it too loose or too tight; you should be able to get two or three fingers under it comfortably, obviously that depends how large your fingers are.
As pp said don't buy anything too expensive as puppy will grow out of it in a few weeks.
When it comes to buying a harness you will either have to try it on, a good pet shop will do the fitting for you, or you can measure your puppy and and order it online.
Again, don't buy anything too expensive until puppy is fully grown as they'll be too small very quickly.
I've kept all my dog's collars and harnesses and it's amazing how tiny some of them are.
Oh and it's not a bad idea to get a tag for both the collar and harness. My dog rarely wears her collar in the house and I often don't put it on when we go out on walks so it's handy if there's one on her harness.