My main Mac is a Late 2012 iMac.
You could let us know what OS you are currently using, and what you do with your Mac?
This totally depends on your own usage. 8GB is plenty for some, really low for others.
You can use this guide from Apple to determine if more RAM would be appropriate for you:
In Activity Monitor, view the amount of system memory being used on your Mac, including physical, compressed, wired, and app memory.
support.apple.com
If you let us know what apps you typically use or plan on using, we can give suggestions.
Myself and just about anyone with knowledge of Macs will suggest you to use an SSD. This alone would be a huge upgrade for your, especially if you are currently using Catalina.
You don't necessarily need to open your Mac, but that would be the cheapest way to get a fast SSD with TRIM support. You would get about 600MBps read speeds using this method.
The next cheapest and easiest way is to use a SSD over USB3. The downside of this is that it won't be as fast as other options, and it will not have TRIM support. But, it is really cheap. In the US, you can get a 1TB SATA SSD and a SATA/USB3 adapter for about $90. If you get a cheap enclosure, it might cost you $100. Speeds would max out about 400-450MBps for read.
Another option that I am currently using is a TB3 NVMe SSD. This is by far the most expensive option, as it requires aa TB3 enclosure, NVMe, TB2 cable, TB2/TB3 bidirectional adapter, and something to power it such as a TB3 dock. The upside is that the TB3 NVMe SSD is the fastest option for the 2012 iMac and it supports TRIM*. You would see speeds up to about 900MBps read.
*EDIT: a TB3 NVMe would be the fastest "single drive" option for the Late 2012 iMac. It is possible to have faster drives using striping RAIDs.
The price tag of the TB3 NVMe set up wouldn't be so bad if you consider future proofing, and/or have plans to purchase a newer Mac in the next few years to utilize the super fast speeds once you make that purchase.
The easiest way to start off is just getting a SATA SSD with a SATA/USB3 adapter or enclosure. This is a good method, because you can always decide later to install the SSD into your 2012 iMac. It is a cheap, easy way to have a huge performance gain on your Mac without having to open it.
BTW, if you plan on doing RAM anyways, I would highly recommend swapping the HDD for a SSD. If you are not doing RAM, then using a cheap external option would be fine.