oxidation is loosing of electron so because Li readily forms Li⁺ on loosing one electron thus is a reducing agent. Alkali metals fall in the 1st group of periodic table as they have 1 electron in their valence(outermost) shell. Hence +1 oxidation state.

Why does potassium only have 1 oxidation state?

Potassium's oxidation number is +1, oxidation numbers are just hypothetical charges that an atom would have if the compound is ionic(This is not always the case),but it's useful for keeping track of electrons. In this case it means that the Potassium atom losses one electron when forming compounds.

Why do alkali metals tarnish?

Alkali metals tarnish in dry air. They form thin layer of oxide on their surface,which in turn react with moisture to form hydroxides. Alkali metals burn vigorously in oxygen forming oxides. ... Lithium is the only alkali metal to directly form oxide and nitride from air.