I used a digital voice recorder I got at Best Buy for about $60. You can use this to practice your voice as much as you like without worrying about being embarrassed or freaking out and losing it- because no one will hear it but you. This helped me develop my voice better than any tutorials and I could get instant feedback by listening to myself.
You already know what female voices sound like- you hear them every day in real life and on TV, radio, etc. Listen to how they sound, and then try to say the same words in the same way, same pitch, same intonations. And it's more about your inflections than your pitch, unless you just have a deep bass voice like James Earl Jones or something. Talk from the top of your lungs instead of from your diaphram. Most of all remember you are now a female speaking- don't let all those old "rules" of how men and boys are supposed to form and speak their words to avoid sounding "gay" get in your way. These are subtle aspects of learned (male) speech but they must be unlearned.
I believe that just about anyone can learn to speak, speech impediments can be overcome, even thick regional accents can be overcome. So can sounding like a man, it just takes practice and determination.