Welcome to RadioReference
There is a large number of helpful articles in the Wiki here, and a number of videos available on YouTube. (A number of the Wiki articles include links to related video.)
Start with the Wiki. On the
Home page, you have a number of suggested pages for newcomers to scanners.
Since you are purchasing an SDS100, look at the links on the
Easier to Read Manuals page.
The Wiki page listing some of the features and capabilities of the SDS100 can be found here:
wiki.radioreference.com
Sentinel is the software provided by Uniden, to update the main database used by your scanner. A link to download Sentinel can be found here:
BCDx36HPSentinel < UnidenMan4 < TWiki
You can install & use Sentinel, to familiarize yourself with it, and if desired, start putting together a Favorites list.
When you receive your scanner, it's memory card will have copies of both Sentinel & the database. However, these may be out of date. You would be better off downloading and installing Sentinel from the link above, then use Sentinel to update it's database. Database updates are usually available by mid-morning on Mondays. and include changes from the previous week.
You will also need to install the Serial driver to connect your scanner to your PC. That can be found here:
(That link references the 996P2, but the same serial driver is used on a number of scanners, including the SDS100.)
Also take a look at this page:
wiki.radioreference.com
The Wiki pages have links to other articles on this and other sites. When you have time, take a look at those that interest you.
If you have questions about specific systems you want to monitor, be sure & indicate your location, as well as the system's link in the database. I would also suggest that you add your location to your profile. City, or County, and state will suffice.
Enjoy your scanner!