I am sure there are tons of ways for a child to learn sign language, so please do not take my suggestion as the best one for every child. I think it has worked beautifully for L. We have the following DVD's (you can click on the links to order them if you'd like, or to look at other possibilities).
Baby Signing Time Volume 1 DVD
We invested in these DVD's one by one. L and I sit and watch them together. The signs are incorporated into songs. I think they work so well for L because she is SO music oriented. She loves, loves, loves songs and hand movements that go with those songs. I think it helps her remember the signs. Watching it with her allows me to learn the signs as well, and then I try as much as possible to use the signs that we have learned whenever I say the corresponding word. I work it into our everyday speech. Also, when she points to an item that we know the sign for, I reinforce the sign by saying and signing whatever she is pointing to.
I want to note that if your child is not doing the sign exactly right, that's okay! L's hands are not perfect in demonstrating her signs, but she is consistent so we know exactly what she is saying. And as her fine motor skills have gotten better with age, she has been fine-tuning those signs as she goes. It will come - the best thing to do is to keep encouraging!
If you are new to this, I would recommend buying one of the DVD's, or renting one of them and giving it a try with your child for a few weeks. If your child seems to be picking it up well and it works for you too, then go ahead and invest in the other DVD's.
I hope this helps. Obviously, do what works for you and your child. There are so many options out there - notecards, books, youtube videos, DVD's. Explore your options! And if you end up trying these, I would love to know how they work for you!
Note - this post is not sponsored by anyone! This is just my way of sharing what has worked for L and our family.