You need jdk1.7 + to run tomcat 8, if you are using jdk1.6, you need tomcat 7-.
See this: http://tomcat.apache.org/whichversion.html
This make me remember back in collage, it took me 2 days to finish jdk/tomcat hello world, :) , what a good time it is.
Just make sure, open a command line, input java --version
to check whether you jdk is configured well, and which version it is in use. Just to be sure, in some case you need to re-login or reboot to make your new configured jdk work.
Believe me, if you configured jdk well, you don't need to change anything of the tomcat, just unzip it, and start it, it would work, unless the port 8080 is already taken.