All you need to clean anything should be within a topic below:
The first thing to recognise about suites is that various areas get far more wear (and consequently dirty) than others so here's a list of where to look and how to start cleaning.
You should always look to remove specific stains before you begin cleaning the whole suite. Once this is done you can just go ahead and clean everything the same way instead of being distracted by having to stop to treat stains.
Arms of suites not only suffer from specific stains, like drink, chocolate or even biro or crayon marks, but also suffer from being rubbed all the time by your arms and this passes on grime. They can consequently get very dirty and often benefit from spraying with a general cleaner before final cleaning, just to help the grime away.
The majority of marks here are caused by heads rubbing constantly in the same area. Padding the area with white spirit can often help because of the grease content in hair, otherwise make a dilute mixture of washing up liquid and brush on, removing a wand cleaner and plain water.
It's surprising how often something gets spilled and the main spillage is cleared up but the drips down the sides aren't. This means that some quite old stains are left and have to be dealt with when you clean the whole sofa. It's also an area where cigarettes often fall and burn the fabric, so any light to dark brown small stains could well be burn marks.
People kick the front panels of sofas, not on purpose but they do it anyway, so look for shoe polish stains here particularly. Also often any spilled liquid such as coffee or tea splashes up onto the panel so be on the look out for light brown stains.
Clearly anything spilt onto a sofa is likely to end up on the top of a cushion. In particular you should expect chocolate, tea, coffee and biro marks.