In calcium shell 'm' has only 8 electrons while a max. of 18 is possible. There is also an 'n' shell with 1 electron. Why?

According to one of the rules of electronic configuration, the maximum number number of electrons that can be accommodated in the valence shell is 8. Thus, M shell can hold a maximum of 18 electrons, however if it is the valence shell, the it cannot hold more than 8 electrons. 

The concept of electronic configuration that you study at this stage is just a rough idea about how electrons are arranged in an atom. Actually each shell is further divided into subshell, and each subshell contains a given number of orbitals. There are four different types of subshells - the s subshell, p subshell, d subshell and f subshell. Each subshell has a specific number of orbitals, and so can hold a given number of electrons. Another important point to note is that these orbitals are filled in the increasing order of energy. It is because of this order (in which the atomic orbitals are filled) that the electronic configuration of of calcium is 2, 8, 8, 2 and not 2, 8, 9, 1 or 2, 8, 10. The rule that the valence shell cannot hold more than 8 electrons will also become clearer once you become familiar with the concept of subshells and orbitals.  

You will learn more about atomic structure and electronic configuration in grade 11, where a full chapter has been devoted to this.