Visit the local bike shop and look at the comfort, commuter, or hybrid bikes. The shop will fit a bike for you or get a frame that is about 65% of your actual inseam. Get a good lock and never forget to lock it. Check the local Ebay and Craig's list for a good used shop quality bike. Stay away from Walmart and other department store bikes. Look for a 15 inch frame, Forge has a nice bike for $230. If you are a AAA menber you can get a 10% discound. I have two Forge bikes and they are shop quality.
http://www.target.com/Women%E2%80%99s-Forge-Athena-V-Comfort-Bike/dp/B0025ZANHI/ref=br_1_5?ie=UTF8&frombrowse=1&searchView=grid5&searchNodeID=125446011&node=125446011&searchRank=pmrank&searchPage=1&searchSize=30&id=Women%E2%80%99s%20Forge%20Athena-V%20Comfort%20Bike