Aiwass, I know exactly how you feel!
It seems very daunting before you go, but generally it should be really relaxed cool vibes, people are almost always very friendly, nobody wants to hurt each other, rather learn together.
This may depend on the individual school, so here's my suggestion: Read up online reviews of schools in your area if there are multiple ones, call them and ask if they have a free trial.. So go there and check it out, check more than one out if you have the opportunity, see how the atmosphere of the place feels and decide where to train. Jiu jitsu attracts a lot of nerds, or just normal people, it should feel friendly. If vibes are weird or anyone treats you bad, go elsewhere. I've trained in like 8 different schools in 4 different countries and only one really was unfriendly, generally it's all good.
By the way, people who eventually get to blackbelt, its not that they have a special ability nor that they were great to begin with.. .Everybody starts being crap at it, like you will, and getting tapped multiple times. The good thing about jiu jitsu is you can always tap out whenever you feel uncomfortable or in danger, and people will let go. As some blackbelt said: "it's not about who's good, it's about who's left". So just start going and then keep going, and you'll eventually be one of them killers on the mat
Let me know how it goes when you do try it out.. A while from now we will talk again and I'm sure you'll be so stoked for having started it, and will be asking yourself why didn't you start sooner