Technically this system only supports upto macOS High Sierra (10.13.6)
While there are a few hacks which you could use I really don’t recommend them if you are dependent on this system for school or work. You also won’t get many of the major features as there is dependency within the hardware. I recommend you stick the either Sierra or High Sierra.
Here’s the guide to upgrade your RAM - [guide|6438]
To add a SSD I would add it as a second drive following this guide - [guide|6634]