Because Ukrainian does not have definite or indefinite articles, either "I am the waitress" or "I am a waitress" would be correct translations.
It's a bit more complex than this.
Ukrainian doesn't have articles, but we have other ways to mark which things are introduced for the first time, and which are known to the speaker: word order and intonation.
With netural intonation, we place known information in the beginning of the sentence, and new information closer to the end. So, «Я офіціа́нтка» is a sentence about 'me', the discussion was about 'me' before, and I tell where I work. This sentence is much more likely to be translated «I'm a waitress». (Linguistically speaking, «я» is the topic, and «офіціантка» is the comment here.)
On the other hand, «Офіціа́нтка я» or «Офіціа́нтка — це я» is the sentence about some waitress (introduced before), and the speaker wants to say that this waitress is indeed her. So, this sentence would be translated «I'm the waitress». («Офіціантка» is the topic, and «я» is the comment.)
You could use a different word order if you emphasise new information. «Офіціантка я» with «офіціантка» emphasised by intonation would still mean "I'm an officiant". However, this makes the sentence emphatic. For example, you could use this emphasis if you want to contrast a wrong idea («Ви менеджер?» — «Офіціантка я.» 'Are you a manager?' 'I'm a waitress.') or to show irritation.