You can use all of the leaves, but generally the fresh young ones gathered before the plant flowers have the best flavour.

Here is a link that might be useful: making herb teas.

You can use any above-ground part of the peppermints to make tea.

FataMorgana

Besides making tea, which is really good with chocolate mint, it is good with mint chocolate chip ice cream or even just vanilla :)

I like to take a sprig, just bruise the leaves and drop it stem and all in a glass of Ice water. It's so refreshing!

JoJo