Here are some basic instructions to install Java 7 on Xubuntu 12.04 LTS:
cd /opt
tar -xvzf /home/marcelom/Downloads/jre-7u21-linux-x64.tar.gz
chown -R root:root jre1.7.0_21/
update-alternatives --install "/usr/bin/java" "java" "/opt/jre1.7.0_21/bin/java" 1
update-alternatives --set java /opt/jre1.7.0_21/bin/java
java -version