First RAM is not user upgradeable, if it can be done at all on your machine the computer must be completely disassembled. In addition a RAM upgrade will not help. You likely have a 21.5" with only a 5400 RPM hard drive which is the primary bottleneck. The ONLY method of making your computer faster is to introduce an external SSD. To learn how, click How to setup an external SSD as your startup disk. The SSD I would recommend is the https://eshop.macsales.com/shop/owc-envoy-pro-sx
However, rather than spending money on a soon to be 4+ year old base model Mac you may want to consider replacing it with a new Mac. You have lots of choices, a new 24" M3 based iMac, a Mac mini or even a laptop such as a MacBook Air or you can even get an Apple refurbished Mac. Refurbished Macs from Apple have zero downside, they are identical to new, have the same warranty as new, are eligible for AppleCare and can save hundreds of $$. If you want to look go to www.apple.com and navigate to the bottom of the page and click the Refurbished section in the Store.
In short you have many options available to you.