I concure with the advice given. You can read over the posts in the shotgun forum for more details on fit, etc..
If looking for a left handed model, that will really narrow down the models, as many are not made for left handed people.
You could use a 20 ga in pump, o/u ,etc. but as mentioned if in the 12 ga, you should consider a 12 ga gas operated which has lower kick. Also, being because of your petite size, posssibly a youth model gun would fit you better than a standard as the stock is shorter. Like mentioned, a good gun store can steer you right on selection and fit for you. Good luck shooting this Falll