In the exam you are given a Periodic Table, so you do not have to memorise every symbol. It shows the symbol of each element. The first letter of a symbol is a capital letter and any second letter is a lower case letter. So Co is cobalt, but CO is a compound of carbon and oxygen. You should be able to use it for the first 20 elements, the elements in Groups 1, 7 and 0, and other common elements such as iron, copper, zinc and silver.
The table also helps you decide how an element is written in an equation. Group 0 elements such as neon and argon exist as single atoms, so they are written Ne and Ar. Hydrogen, nitrogen, oxygen and the Group 7 elements exist as diatomic molecules, so they are written H2, N2, O2, F2, Cl2, Br2 and I2. Metals are written as their symbol, for example Na or Fe.
Worked example: sodium burns in chlorine to make sodium chloride. Chlorine is in Group 7, so it is Cl2. Unbalanced, the equation is Na + Cl2 → NaCl. There are 2 chlorine atoms on the left, so write 2NaCl, which then needs 2 sodium atoms on the left. The balanced equation is 2Na + Cl2 → 2NaCl.