Margin: The white space around the edges of page.
Justification: Making all the lines the same length (so the edges of the text are straight on the right and left).
Proportional: Some fonts are proportional -- the amount of space a word takes up depends on how wide the individual letters are. Non-proportional spacing is when a word with, say, five letters always takes the same space on the line.
For example:
Non-proportional:
iiiii
wwwww
These should all be selections in your wordprocessing program.
In general, your manuscript should look like it was produced on a typewriter:
Non-proportional, not justified, and with one-inch margins all around.