Each child develops language at her own rate. Typically children say their first word around one year of age and then slowly acquire more words. Some children can say around 70 words at 18 months, however others take longer to get started. The key is that your child's receptive language, meaning what she understands, is not delayed. A typical child at 18 months can follow directions, point to a number of pictures in books, point to objects/people in their environment when asked, and point to several body parts.
Reading to your child makes the biggest difference in language development and future reading skills. Also, imitate and expand on your daughter's attempts to speak. If she says, "Uh-oh," you say, "Uh-oh, we spilled the milk!" If she still isn't talking by her second birthday, talk with your doctor and consider an evaluation by an early intervention specialist. This may ease your concerns if you continue to have them.
Also see the Reading Rockets section on developmental milestones for speaking and reading.