So it is as well to make sure you understand the difference between the terms shell, sub-shell and orbital.
A shell is all the orbitals/sub-shells found at approximately the same energy.
So, if an element has an electronic structure' of 1s2 2s2 2p6 3s2 3p5 we would say that the first shell is made up of the electrons denoted by 1s2 because only this orbital has a quantum number of 1. Since the 1s orbital is all there is in this shell you can either choose to think there are no sub-shells or that the s sub-shell is the shell.
The second shell is made up of the electrons denoted by 2s2 2p6 as they are all in orbitals with a quantum number of 2. In other words, this shell contains two sub-shells (s and p) and a total of four orbitals and 8 electrons.
The third shell is made up of the electrons denoted by 3s2 3p5 as they are all in orbitals with a quantum number of 3. In other words, this shell contains two sub-shells (s and p) and a total of four orbitals and 7 electrons.
In short sub-shell means all the orbitals at the same energy. Each s sub-shell has 1 orbital, each p sub-shell contains 3 and each d sub-shell contains 5.
You are very unlikely to be asked to define what is meant by a shell.
So, you only really need to know that electrons in shells with lower quantum numbers (nearly) always require less energy to be removed (ionised).
And that shells do not always correspond to periods because the fourth period contains 3d orbitals etc.
You should probably be aware that the lower the quantum number of the valence (outer) shell the smaller the atom and the shorter and stronger any covalent bonds it forms will be.