can I add more memory in my desk computer?

You can add memory, but an SSD replacing your rotational hard disk will make a bigger difference in the speed of your Mac.

Even if you get an external USB3 SSD it will still make the iMac faster. You just use something like SuperDuper <https://www.shirt-pocket.com> to clone your boot disk to the external SSD (SuperDuper full clones are always free) and then use System Preferences -> Startup Disk to point at the external SSD as the boot device.

What model Mac do you have, exactly?

8 GB of RAM is standard and it can be upgraded at the time of purchase to 16 GB for an additional US$200. Although Apple does not intend for one to upgrade the RAM after purchase, and doing so is difficult, it is possible.

It isn't easy I'd watch the Install Video first...

How to Install Memory in 21.5" iMac (2012-2014) (macsales.com)

16GB Memory Upgrade Kit for 2012-2013 iMac 21.5-Inch (macsales.com)