What to use to Skim Wall and Fill Gaps around Cistern?

[Jewelz]

I am a complete novice but need to skim some walls in my toilet. The walls are pretty uneven and have shallow holes, I would like to smooth them over. As a complete novice should I use a one coat plaster? Or could I use a joint compound? I have seen that this might be easier to use for a novice.

Also, I have some fairly big gaps around my cistern which need to be filled, what's the best way to do this, how to I get to them?

Would appreciate any advice please, Thank you.

Thank you