Unfortunately for this intake there will not be a "typical shift pattern." At present station staff are either classed as 'rostered' or 'reserve,' and you will go in as reserve.
Rostered staff, as the name suggests, follow the roster and can work out indefinitely (until the roster is changed) what shift they will be working on a specific day or when their rest days will be. They are only rostered to work at one station, although there are few cases where you may work at a nearby station although this is usually for the cover of rest days or meal reliefs.
Reserve staff are informed of their rest days and their rough duties four weeks in advance, and the finalised duties are confirmed by Thursday for the week commencing that Sunday (although you can still be requested to change your duty by two hours either way with 24 hours notice). Note that some station groups operate a rest day roster system that means staff can find out when their rest days will be for some time in advance, although this is not for all locations, and even if you work at those locations that operate such a system, some weeks will be flexible when you will not find out your rest days until nearer the time. Reserve staff could work at all stations in a group.
I will not go into the specifics about over establishment staff as that is perhaps a bit too complex but see about reserve staff above.
As to the number of weekends, off that is really dependent on where you work. There are some CSA rosters, for example St James's Park, where you only work two in every five weekends. There are other locations, such as Knightbridge, where you get two in nine Saturdays, and four in nine Sundays off.
