The first thing to understand is to get an idea of size: size of the known universe, size of a cell, size of an atom
Enjoy this very old film – if you find anything better please send me an email!!!

Diffusion and osmosis first to understand anything from Khan Academy

The cell as explained by Bozeman Science

And the plant cell explained by Hank Green in Crashcourses

complemented by Ted Ed

and animal cells by Hank Green’s team

and then a funny informational video of what the nucleus contains: