"I am a woman."
Translation:Jag är en kvinna.
Omitting "en" in "Jag är en man/kvinna" is done by us natives in some informal situations like in a dating profile. (incorrectly or not, I'm not a grammar expert) However, I can't think of a single situation where omitting it actually sounds better but many where it would sound obviously wrong, so just learn it the proper way. :)
Even if someone explicitly asks you: "är du man eller kvinna" you can/should still reply "jag är en kvinna" (or just "kvinna").