To add an indicator of RAM or CPU usage, I recommend installing a system monitor extension.

To install the extension, just run the command:

sudo apt-get update && sudo apt install indicator-multiload

Now you can find the system load indicator in your applications list.

Select it, and you will see the small indicator at the top right corner (top bar).

Click on the indicator and select Preferences.

Select indicator items:

Add a new element:

RAM $(size(mem.user))

As a result, you will see:

Make sure that your new indicator item is on the top of the list.

That’s it. Now you can easily see the indicator of RAM usage in the top bar of your ubuntu machine.