You should revise on these O Level concepts and skills first: Mole concepts and forming balanced equations. This is because these concepts and skills will have to be used across almost all the topics in A level Chemistry.
Also, the first four topics of A Level Chemistry: Mole concept, redox reaction, atomic structure and chemical bonding will lay the foundation for the subsequent topics in the subject. Hence, my advice is to read up on some of these concepts before you start your JC journey. Important concepts include the spdf configuration, hybridisation of orbitals, intermolecular forces of attraction between simple covalent molecules (id-id interactions, pd-pd interactions and hydrogen bonding), molecular geometry.