Don't feel embarassed. We hear this question all the time.
DO - Pronounce Geist with a hard G, as in Growl.
DO - Pronounce the ei as a long i, as in Rile.
DO - Geist rhymes with Heist, as in stick-up, so that Geisty, if it were a word, would rhyme with feisty.
DO - It’s all right to think of Poltergeist.
DON’T - Definite no-no: Geest, as in Canada Goose
DON’T - More no-nos: Gaste, as in make haste; Jyst, as in widening gyres; or (especially) jaste, as in as the driven snow.