Hello, I suggest you start with the basics of OpenFOAM. You can find lots of resources on the official pages. See https://www.openfoam.com/documentation/user-guide or https://doc.cfd.direct/openfoam/user-guide-v11/index. There is many more when you google it.

As regards the solver, for your application you could use some of the multiphase solvers, e.g. interFoam. Look for a dam break tutorial, it could be the right solver for you and then adjust it to your needs.

Good luck! Jan