This is not an easy problem to solve. It's possible to shift a signal's frequency but there is also the timbre (which is a huge topic in itself) and intonation to account for. I have a mixer and several plugins which will shift the frequency of my voice, but none of them make me sound female.
I guess it all depends on how female you want the output to be. If it has to fool real people then it could be hard to do.
It won't be an easy project as Torben mentioned. Google for guitar effects schematics for pitch shifter, octave shifter, cry baby, wah-wah or wah pedals. These effects won't make you sound "female" but will make a blurrier, higher frequency replicate of your voice.
I have noticed that today's male pop singers sing at female pitches that are so high that I can't hum along with unless I hum at an octave lower.
I'm not talking about Michael Jackson.