Babies love music, so, no matter how poorly you do it, sing — you will have an appreciative audience. Turn on the stereo to music you like. Your baby will probably like it too, whether it's rock or reggae or classical. You don't need to buy special tapes of baby music (unless you have a particular fondness for the sounds of Sesame Street or the Barney song). Down the road you'll be listening to your kid's favorite music often enough, so now's your chance to get her to listen to your favorites.
If you have a wind-up music box (on a mobile, in a stuffed animal, or freestanding) you'll use it a lot. You'll use it now to entertain your baby in the crib or on the changing table, and later when your baby figures out how to wind it up herself.
You'll also be using musical instruments for a long time. Your baby's first instrument will be a rattle, and initially you'll shake it for her. Try shaking it on one side of her head and then the other. After a while she'll start looking around, trying to figure out what is making the sound. At about four months she'll be ready to hold the rattle for herself. At six months or so she'll start banging rattles together to hear the different sounds that makes. At this point she'll be ready for more complex musical instruments — like drums or tambourines.
Besides music, introduce your baby to animal sounds. If you hear a bird or a dog outside, imitate it. If you look at a book about animals, make all the sounds yourself.