You have already tried to fill the shoe by using the illustrator "fill".
But your shoe is made up of seperate lines. Illustrator looks at every line as a different shape.
You can merge all the lines into a single shape and then fill might produce the correct result.
You can select all lines of the shoe and then use the Pathfinder tool to create a single shape from the lines.
Alternatively look at the "Object" Menu. There should be tools for merging multiple shapes into one.
(I am not sure how the tools in there are called, sorry)
Alternatively you can create a shape without border and put it below your outline. Then you fill this single shape. Your shoe will then have a color. You can create multiple of these for various colors if you want to.
If you do the same for the pants and then put the shape above all of the shoes layer this will effectively hide them.
Partitally hiding shapes
If you don't want to use the color shapes or you don't want to color at all then you might want to consider using layer masks.
These are quite easy to use and you can find tutorials for them online.
Layer masks hide parts of a shape or a group (your shoe is a group of lines).
With them you can hide the parts of your shape which should be hidden by the shoe.
This is better than erasing because you don't loose any drawings in the process. You can later adjust the layer mask to show more/less of the already drawn object.
Personally i would've used a layer mask. I think the lines/color/shadows should be on seperate layers but this is mostly preference and everyone can manage their layers differently.