As you know RAM is temporary storage. When you are running windows it wil take some space on RAM, and there some softwares or application that by default run in background which they take some space too. And any open programs will take some space in RAM too.

So by closing unused programs you will free up some space of RAM, by closing background applications using Task Manager which can be opened by holding ctrl+alt+delete all together you free up some other spaces.

And you can find third party applications to do that for you.