I agree with the suggestions to read up a bit on articles on how to pick a dropper, as well as to look at PNW. I recently picked up my first dropper and was similarly perplexed. I confirmed I had internal routing, found the measurements following some articles, then looked at different droppers/companies and found ones for prices I was willing to pay. The one I liked the most was a PNW, so I then messaged their support team to confirm my selection would work with my bike and get some advice about the right size. They were very helpful.
About installation, you may have received a manual or can find one online that may provide some helpful information you need. I attempted installing mine, but found I had to remove the bottom bracket and rear triangle in order to angle the internal routing up the seat tube. I didn’t have the right tools and it took me out of my comfort zone, and it took me a while to get to that point. I ended up calling my local bike shop and they were willing to install it in the $25-35 range, which I ended up doing. (Money is money, and that can be significant added on top of the price of the dropper, but it was way more reasonable than I expected and I had spent a decent amount of time to get to that point already.)
So don’t be afraid to try it out yourself and then ask for some help at the shop. I wouldn’t be surprised if they would give you some guidance if you get stuck. And if you end up taking it in after working on it, remember that you’re not the first to do that and won’t be the last. 🙂