The pressure is meant to be between 1 and 2 bar (or in the green zone if it's coloured). Usually under your boiler there is a filling loop which is about 15 cm in length (it will be shaped like a square arc) with two blue or black valves/taps. Turn them both until the pressure reaches about 1.5 bar. Turn them back off. (There are two taps/valves because regulations state that there must be a way of cutting it off from the mains water supply.)
If the pressure doesn't stay after you turn on your hot water/heating, you need to check every one of your radiators. If there's a leak, or it's cold at the top and hot at the bottom, you'll need to either bleed them or tighten the ends.
If there's a leak, you'll need a flat head screwdriver or a radiator key to tighten the bolt (usually on the top right hand side of the radiator).
If they're cold at the top and hot at the bottom, you'll need to bleed them. Use the radiator key or flat head screwdriver to undo the end slightly. Have a towel ready underneath. You should hear air coming out. Once water is flowing out, you need to quickly tighten the end again.
If the pressure on the boiler drops, you'll need to add more water to the boiler (see first paragraph).
Your boiler's manufacturer's website should have information relating to the specific location of the filling loop.