TL;DR (the specific answer)
Your voice part's key signature has one flat. The corresponding alto saxophone key signature will have two sharps.
The general answer
Voice is written in what is called "concert pitch". Concert pitch means that when you write an "A" (specifically, the "A" above "middle C") it is tuned to 440Hz. Instruments like voice, piano, and flute — so-called "C instruments" — are all written in concert pitch.
The alto saxophone is an "Eb instrument". This means that its written music and the "concert pitch" sound that results are not the same. When "C" is written in an alto saxophone part, the resulting concert pitch — the note a singer would sing to match the alto saxophone's "C" — is the Eb below.
Put another way, concert pitch — the actual sound produced in response to a written saxophone part — is a "major sixth" lower than the written note. And to produce a given concert pitch, the saxophone part must be written a major sixth higher.
Thus: D minor / F major becomes B minor / D major.