If you want to put an SSD in place of a hard disk in a desktop PC here is some info.
SSDs are more expensive per GB than a hard disk. So a 1TB desktop hard disk may cost you only around £35 but a 1TB SSD may cost you around £100 (prices do vary across brands of course).
However SSDs have no moving parts and are faster than hard disks so it is an excellent upgrade to do. Maybe buy a 240GB SSD (roughly £30 or £40) to hold Windows (for the speed) and use a "normal" hard disk to hold your personal files.
Hard disks in desktop PCs are 3.5 inch (physical size) but SSDs are 2.5 inch (physical size). This means the same SSD can go inside a laptop as can go in a desktop PC.
However the bay or slot that holds the hard disk in a desktop PC is 3.5 inch in size so to fit a 2.5 inch SSD you normally need to buy a small bracket to fit on the SSD so it fits inside the 3.5 inch desktop "slot". Plenty of these available on ebay or Amazon.
Note the cable connections to an SSD are the same (SATA power cable and SATA data cable) as a "normal" desktop hard disk.
If you have two hard disk slots or bays in your desktop PC you can put the hard disk in one and the SSD in the other then use some software to copy or clone Windows 10 and all the files on the hard disk to the SSD. This is not difficult to do but does require some technical knowledge.
The other option is to put the SSD in the PC and do a clean Windows install (which may be a better option).
Below is a photo I have just taken that may help. I have placed a 50p coin in the photo to give an idea of scale.
It shows a full size 3.5 inch desktop hard disk on the left
It shows an SSD in the centre as you may buy it. As you can see it is much "thinner" (width) than a 3.5 inch hard disk
On the right it shows an SSD fitted inside the bracket you need to buy to make the SSD fit inside the 3.5 inch slot (there are numerous types of brackets). As you can see, with the bracket on the SSD it is now the same "width" as a 3.5 inch hard disk so will fit in the slot.