all three of mine knew sign language by 15 months of age.. i found it the easiest to teach them sitting in the floor singing songs while signing to them (sorta like you do with itsy bitsy spider) seems to work because its fun for them
I have been using the basic words milk, eat, sleep, with Kaelynn for a few months now. She signed milk for the first time yesterday morning and she used it everytime she wanted to nurse. They say they can sign when they can learn to wave.
Good luck.
I used this with my middle three. It started with DD7 as an infant because she wasn't hitting her verbal milestones. It helped with her a good bit, then we just used it with DS6 because DD7 still wasn't talking. Then with DS2 it helped with him, even though he talked pretty much on target. I think it's great for kids to learn sign language, even just basic. If it's something that is kept up with (unfortunately we didn't) it will become very useful later in life. I know a lot of the kids channels are now doing a lot of things with sign language.
I'd personally recommend American Sign Language. Some of the items marketed as "baby sign" are ok. What ever method you use just be sure to use the verbal English word with the sign so your child will connect the spoken word with the sign. If you're asking if she want's an apple, show her the apple while saying the word and signing it. She'll pick up the signs first and then she'll pick up the speaking.
http://www.aslpro.com/cgi-bin/aslpro/aslpro.cgi
http://www.lifeprint.com/asl101/pages-layout/concepts.htm
http://www.handspeak.com/word/index.php
You just have to be very persistent about, and do it every day all day. Since its a form of language she will be learning, she needs to be able to see it and practice it at the same time. Its just like how you speak to her and then she babbles, well then she will try to mimic the same hand gestures you do as you speak to her in that way.
here in the UK we have a kids show called 'something special' with a character Mr Tumble who is a clown. he says the word, does the action and repeats the word then asks the children to try & gives children the time to copy. cleo picked up a lot with colours, actions and food when she watched it. i do not know sign language but would love to know how to do it so i would try learn what she would learn so i could understand her. if you get a chance it may be on youtube & if you think its worth it im sure it could be bought on DVD or something