18 month old only saying a few words... concerned.?

My daughter is 18 months rarely talks.. when she does, EVERYTHING is "ba!" If she sees something she wants.. "BA!", or if she sees Dora on TV, "BA!". She does say other words like hi, or dada, rarely mama. Is this normal? I have been reading about all my friends kids saying other words early. My daughter understands me when I ask her to go get things or simple things like sit on your bum, get your boots. Am I worrying for nothing? What things should I do to try to get her talking more?