Phonemes are sounds, and graphemes are how we write them.
Phonemes can be written by a single letter grapheme, or by a digraph (2 letters) or trigraph (3 letters).
In Reception, children will learn one way to represent each sound.
As children progress through the different phonics phases (into Year 1 and Year 2), they will also encounter more ways to write/pronounce different phoneme-grapheme correspondences.
Don't forget, sounds should be 'pure' (no "uh" on the end). The video below will help.