My Advice..
Find a local range that rents firearms for use at their range, experiment with as many as possible in your price range for a session (and don't be afraid to do a few times at the range to expand your horizons) and find a firearm that feels "right" to you for use. I would never presume to advise anyone to use a firearm that I prefer, my hands, and my feel of a weapon is far different than other folks I'm sure. And certainly different than what a female would prefer.
I carry a Springfield XD-9, it feels right to me. My wife on the other hand has an issue with "racking" the slide of a semi-auto, her preference thankfully is a older model Ruger Security Six in .38 Special that I've had for years. Funny though, our son has a Bersa UC-9 that she can rack the slide perfectly, and in the times I've fired his firearm, I've found it to be a great shooter. I like it as much as my XD, and the sale price was pretty reasonable, around $360.00 as compared to a XD selling today for at least a hundred more. I was lucky and got my XD in the $360.00 range last year.
If you find a weapon you prefer, you don't have to buy new, pawn shops have a variety of used firearms for much less than a new one.
But bottom line on caliber, in my opinion, for a self defense weapon is a .380 or larger.
Welcome to the forum, I thank you for your service for our country, and give you a salute for coming to the point you have decided that you're responsible for your own safety rather than counting on someone else (or the cops) to assure your well being.
After you buy one, PRACTICE and seek further education in self defense as often as possible. And seriously consider obtaining a concealed carry permit if after looking at the possible scenarios you could find yourself in it makes sense to you.