Shower problems tend to fall into a small number of categories, and the symptom usually identifies which.
A mixer that drips, becomes stiff, or will no longer hold a temperature setting is almost always a worn cartridge. Cartridges are specific to the brand and model rather than universal, which is why knowing the make helps.
Temperature that swings when another tap or the toilet is used indicates the mixer is not compensating for a pressure change, and where that produces sudden hot water it is a scalding risk rather than an inconvenience.
A diverter that will not send water fully to the shower head, or lets water dribble from the bath spout while the shower runs, has a worn diverter mechanism.
Poor flow from the head alone is usually mineral build up in the outlets, which is the simplest of these to address.
If a hot water system is leaking, making unusual noises, or delivering water hot enough to scald, switch it off at the isolation point if you can do so safely and call a licensed plumber. Tempering valves and pressure relief valves are not do it yourself items, and a failed relief valve can be dangerous.
