Reddit reviews Baby Sign Language Made Easy: 101 Signs to Start Communicating with Your Child Now

I do some basic signs with my twins (they are only 4.5 months but i figure starting early and stickign with it is the best way for them to get the idea) and the signs I use are hungry, milk, mommy, daddy, kitty, ball, and done/finished.

I found this book pretty useful as it doesn't just tell you what signs to use and when it also has a lot of stuff on what it might look like if your baby starts to sign back at you because they don't have the motor control to make the exact sign at the same time that they figure out that the sign means something and try to make it themselves.