Gaunce, you are mixing two commands together and mangling one of them.
You need to do two things, independently of one another.
First, cd into the location containing the .jar file.
Secondly, run the jar file.
So, as John VV said:
and then hit RETURN. This should take you in to the folder (whatever it may be). If you do not know enough about the terminal to cd to this directory, try typing "cd" (without the quotes) and then dragging-and-dropping the directory (containing the jar file you want to execute) into the terminal. Its full path will be pasted as text into the terminal, and then you can press RETURN.
If you do:
You should see, in the terminal, that you are now located in the directory containing your jar file.
Assuming you have gotten this far, you can execute the jar file. It looks to me like you got from the wiki link that the command should be 'java -Xmx1G -jar minecraft_server.1.8.jar', so I'll assume you are correct about all those fancy flags (although they're greek to me). So you would execute the jar file like this:
java -Xmx1G -jar minecraft_server*jar
And that should do it.