Well if you did download and isntall the newest JDK (NOT JRE) then you should be able to find id on your system. Check your operating system for the right version (32 bit or 64 bit)..
After that in Eclipse under Window/Preferences/Java/Insalled JREs you can add the JDK definition you installed and want to use. Just click on the Add button and add it, don't forget to check it as the default, then Apply!
If you go then to your project, right click it, go to Build Path/Configure Build Path, under the tabb Libraries you should see your selected JDK version as System Library.
Under Java Compiler you can check the Compiler coplience level: Eclipse Mars has 1.8 as Default e.g.
Hope this helps you to set up your Eclipse..