How to free up RAM space

My computer runs very, very slow; its an iMac (late 2013) computer and I currently have the Mojave os. I seemed to have maxed out my 8GB of RAM. I still have over 700GB of space on the hard drive. I have checked the "Memory" column in the Activity Monitor and 1) the memory pressure is yellow, and 2) something called Google Chrome Helper is listed several times.

Ultimately, I would like to learn how to free up space.