For each shift, you will be given an area to patrol. This may be part of the 'public' station or an area like a delivery yard. You will be responsible for the security of that area, including checking the ID of anyone in non-public areas, making sure that nothing is open that should be closed, and that nothing is closed that should be open and that details of any vehicles are properly recorded.
You will need to remove persons that don't have business in the station, this includes vagrants, beggars, taxi touts, and those refused travel by TOCs.
You may be required to sign in contractors and visitors, as already stated, and give a safety briefing.
You will still be expected to assist customers with directions, train times etc., although for ticketing queries you should direct customers to the relevant TOC.
You may also be given a station tenant to 'look after' and ensure that they are carrying out their security duties to the required standard.
At my station, not all security CSAs are first-aiders, and not all first-aiders are security CSAs!