Let me start with the obvious: this is a 9 year old laptop. So putting time and effort into upgrading it should be up for debate. Especially since you won't be able to re-use any of these upgrades in a new laptop, at least if you stay with Apples current lineup.
The first and pretty much mandatory step for a 9 year old laptop will cost you almost nothing: clean the fan and heatsink, and apply new thermal paste. Here is a video about replacing the heat sink, which might not be necessary for you. But you can still see what needs to be done. https://www.youtube.com/watch?v=tL2TaoomHkc
If you don't feel confident doing this on your own, repair shops should offer this kind of maintenance for 50$ or less. Again, it is debatable whether dropping money on such an old laptop is worth it.
About the SSD vs. RAM question: I have used laptops of similar age with and without an SSD, and the difference is night and day. Not for CPU intensive work like compile times, but the overall "snappiness" of the device is greatly improved. You can use most regular 2.5" SATA SSDs for this upgrade. Samsungs 860 Evo series is a very popular choice, and will cost you around 90$ for 500GB of storage. You can either replace the current hard drive, or ad the SSD instead of the optical drive. Here is a great video explaining the options and the steps involved: https://www.youtube.com/watch?v=pQyKcDpP6kc
And Ifixit has guides for this: https://www.ifixit.com/Guide/Installing+MacBook+Pro+13-Inch+Unibody+Late+2011+Dual+Hard+Drive/13786
And again, if you will ever upgrade to a current Macbook, you won't be able to re-use this SSD.
And RAM: this really depends on your use-case. 8GB is not too shabby for regular use. But depending on the size of your projects and how many applications you like to use/have open at the same time, it might be a limiting factor for you. The easiest way to find out is by monitoring memory usage when you feel like your device is getting too slow. Your Laptop can be upgraded up to 16GB of RAM. The video linked earlier also explains this step. And of course Ifixit to the rescue: https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Unibody+Late+2011+RAM+Replacement/7651
Pretty much every DDR3 SODIMM modules with 8GB capacity should work. Your laptop officially supports only 8GB of memory maximum, but people have successfully upgraded to 16GB.