If you have a sealed combi system your pressure should be topped up to around 1 bar, (consult your manual for the manufacturers recommended operating pressure).
This is done via a silver braided pipe under your boiler in which the valve is opened to allow water through until you reach the correct pressure.
More details here.
Valliant Turbomax has a small key which is turned by a quarter turn to allow water in.
Worcester Bosch has a large plastic key which is inserted and then you open a round grey knob to allow water in. See post no 41
