Make sure you got at least a 8" spreader when doing mudding on drywall. A smaller one will not give you the gradients you want.

If you got a gap in a seam, you mud the gap, take some tape and fold it in half, press it into the seam then mud over the paper to fill in the gap.

You can sand between applications of mud once it dries. Since youa re new to this, allow this to take place over a couple days. Once you get good with it, you will have next to no sanding to do and do it in a single application.. I always run a 12" spreader and I only need to spot sand.