I know that the best way to end a fight is to leave before it even starts, but sometimes that might not be possible.
Well, you seem to have good intentions and a good grasp of the concept of self-defense.
A lot of 'it depends' here - and I'm not expert, so I'm probably not qualified to give expert advice.
But some general thoughts...
Consider where you live. Different areas of the US have different concentrations of styles, I've found. TKD, WC, and various Okinawan karate styles seem to be more prevalent in some areas than others. It always helps to have a place you can actually go nearby, as it increases the chances you'll actually keep going back.
Consider the background of the training center you're considering, including the teachers. Their background is something you may wish to consider, such as from whom they learned, what styles they learned, what advanced belts they were awarded and from what associations or schools. Some careful googling should help.
Consider checking out the local BBB for complaints. You never know.
Check out the dojo/kwoon/etc in person. Ask to observe a training session, and if you get a chance, ask if you can speak to a few of the students before or after training. Ask them what they think of the place, what they feel they've gotten out of it, how long they've been students, anything they wish they had been told before signing up, etc.
One of the things that usually gets me is when an instructor starts out by talking what I might expect in terms of belt level within a given period of time. I'm not saying this is bad, but it gives me visions of belt factories, and it tells me that the teacher thinks I'm primarily interested in getting a belt, instead of getting training. And although there is nothing wrong with contracts in general, I prefer not to sign one - or at least, to have the option of breaking the contract for a minimal fee if I attend for awhile and discover it just isn't for me.
Do some reasearch on the style in question. Most have short summaries on Wikipedia, etc. Each has distinctive traits that may or may not be to your liking or suit your particular body type.
Good luck with your search. It may take some time to find the right style for you - nothing wrong with trying different styles. But also remember that sometimes you just have to give a style some time to get used to it before you can fairly judge it. You have the benefit of time - relax and enjoy this.