I'm beginner in SW and I don't know very well flow simulation so I explain my problem

I have build a tank storage with an inlet and an outlet and I want to smiluate a flow of water through the cylinder tank

But I did the different tutorial of  FS and I don't know how I can impose that there is a fluid (water) in the storage tank

I've tried to create lids in the inlet and the outlet but it don't understand that I want a fluid in the tank.

In the tutorial, th've already create an enclosure so I think that it could help me but I don't how it will be create ??

Please can you help me in easy english...Thx

PS : if you want more details, contact me, there is no pbm !