Unfortunately it's not quite just plug in the frequencies and go. The 15X uses a technology called Dynamic Memory Allocation which requires more than this. We'll get to that in a moment.
Let's start by getting a handle on your county's listings...
https://www.radioreference.com/apps/db/?ctid=1826
The trunked systems at the bottom of this page are incompatible with that scanner, so ignore them. Your police, fire and EMS are all analog; however anything marked with 'M' are mobiles, and you would need to be pretty close to hear them. A better antenna is required for these, but that's another topic entirely.
We have a lot of things in our wiki that will help you along; you should start here...
https://wiki.radioreference.com/index.php/Program_your_DMA_Scanner
For your purposes, a freeware program known as FreeScan will do the job of programming very easily. There's a link (always in blue) for this in the above article. You will also need a USB cable (I'd recommend the USB-1 from Uniden, and available from Scanner Master). Also at the very bottom of this article are links to the Easier to Read guides, including the one for the 15X. This will take you through all the new terminology, including setting up systems and groups (which is all you need in your case). It is possible to program these scanners manually, but be prepared for LOTS of knob pushing. You may also find someone in your area in the
NY forum that can give you a FreeScan file.
Mike