These are pretty basic tasks but they are important things to know if you are new to Linux. These are particularly useful if you are running a Linux system without an X server.
To find the Linux kernel version that you are running, open your favorite terminal or CLI and type in: uname -a. Using the -a switch will display all information related to the kernel version. Usually, uname -r will be sufficient.
To find the version of Ubuntu that you are running, open your favorite terminal or CLI and type in: cat /etc/issue. This will give you the version number only. If you need to know the name of the version just run
cat /etc/lsb-release and check the named value in the field named DISTRIB_CODENAME=.
Hopefully these simple but useful commands will help you.