Let her take the lead. If you notice her looking at a dog, say, "That's a dog. It's a brown dog. The dog says woof." If she seems interested, you can say it again. Next time you see a dog, remind her of the first one. Also, remark on her play ("Listen to that blue rattle! You're shaking it!").
Narrate her day. Describing your routine ("Look who woke up from a nap! We're going to change your diaper now, and then we'll have a snack.") is a lovely way to fill her life with words and to convey how much you enjoy having her around.
Listen. Being part of a give-and-take conversation is key for your baby. Respond to her coos like she's telling you something exciting ("Really?! You don't say!"). Then tell her something exciting, too.
Don't overdo it. Endless prattle won't lead to a Nobel Prize. More likely, your baby will just zone out.