Firstly, make sure everything that should be tight is tight - drain valves, jubilee clips, pipe connectors etc. Then, if you have valves on your skimmer and inlet, open them up to let water into the pump, and bleed the air from it. You also need to make sure the filter (is it a sand filter/new sand?) is full. Set the valve to filtration and start the pump. Water should be drawn through the skimmer, into the pump, through the filter and back into the pool. If it’s a new installation, the filter sand should be backwashed (setting on filter) until the water is clear before filtering back into pool.
If all of that’s good, then get some chlorine in, usually best to use high strength bleach to start with - 20 litre from a brico shed is about €15, maybe put 5 litres in to start to give the system a “shock”. You need a testing kit, basic ones do pH and total chlorine. Ideally 3ppm chlorine and pH of about 7.4. If it’s above or below, adjust using the right chemicals pH+ or pH-).
Hope this helps a bit, Good luck