Meanings
once, one time (one and only one time)
Ich werde das nur einmal sagen.I'm only going to say it once.
sometime (at an indefinite but stated time in the past or future)
first of all, for starters, firstly (as an initial point; before any other considerations)
one order (of food)
Einmal Pommes, bitte.I'll have (one order of) fries, please.
Examples
GermanEnglish
Warst du schon einmal in Kyōto?Have you been to Kyoto?
Er sah sie noch nicht einmal an.He didn't even look at her.