Some good answers so far, but I will add my $.02
First of all, any bike you get, you should be able to sit on it with both feet flat on the ground. This is sometimes a problem for girls, and limits their choices.
Secondly, if you can drive a shift car, you can very easily learn to shift the bike. You already understand how a manual clutch works. I learned to drive on a shift car, and when I bought my first bike I just jumped on it and rode it home. At night! You can also ride a bicycle, right? If not, you should start on one of those to learn how the balancing thing works.
Thirdly, the Ninja 250 is all the bike you will ever need -if- you never plan to go long distances at high speeds. If you plan to do long highway trips, say 30 mi. or more at 65 mph or more, then you should consider a 500. The 250 can do 70 or 80, but for long distances it is not comfortable, and not safe since it's not really designed for those speeds. Years ago, before superhighways were so popular, 250 was the standard size for motorcycles, and people routinely crossed continents on them at 50-60 mph.
Good luck! I hope you enjoy biking nearly as much as I do. 8^)