El Capitan and memory
memory could mean two things. RAM memory will clear up after you restart. And physical memory is used to store things. 

Your Mac sounds young, so it might be a RAM memory problem. Try restart a few times.

If that doesn't work, type sudo purge into your terminal. That will manually free your RAM memory.

Lastly, there is a option to clear up NVRAM memory. If you clear NVRAM up, it will clear up your settings for system volume and mirroring and stuff. The way to do that is restart, after you hear the boot-up sound, hold down following keys: Cmd + option + p + r

To clear out physical memory, in the other hand, is simpler. Move things to usb and delete unwanted apps and plugins.