A:Marin's Kentfield FS ($500) is whats broadly called a city" bike. Its great for daily errands, short commutes, rides that combine bike paths and streets, that sort of thing. Its a nice bike, with a light aluminum frame, Shimano components that work well and are easily repaired or replaced, comfortable upright seating, and plenty of gears for hills. Its a nice...simple...bike.
The Kentfield is great for a beginner, has a comfortable design (although its a bike, and will still take some getting used to), and comes in womans sizes. The difference between the 17-inch or 19-inch models is simply a matter of size, and your bike shop can advise you on what is best for you. Nineteen is kinda tall, so my bet is 17.
There are other choices. I also like the Cannondale Quick Feminine 4 ($600), which has a few goodies the Marin does not (including a carbon front fork, which makes for a better ride) yet still is a very user-friendly bike. Or the Novara Forza (from REI, for $550), which gives you still other options (handle bar ends, which offers more grip positions) in the same basic package as the Marin and Cannondale. If you can, go to a shop that has them all and ride them. The right bike will speak to you.
Then, when you buy it, do what my friend Phil says you MUST do with a new bike: Pedal to the nearest ice cream shop, and order a big cone!