Page Margins and page size are really important parameters if you want to have a neat and nice print of you work.
First lets see how to declare the paper size, that is declared with \documentclass control sequence.
Now, this picture is going to help us know what variables to use to set the margins.
As you can see you need to declare the topmargin, and to declare the bottom margin, you will have to add topmargin + textheight +footheight, and subtract that from the paper size, that is going to be your bottom margin.
It is the same with right and left margins, you just define the right margin with oddsidemargin, have to take into account that there is already one inch (2.54 cm) of margin if you want something less than that you may use a negative value for oddsidemargin, for the left margin you have to add (1 inch + oddsidemarging + textwidth - paper width).
evensidemargin control sequence, is used to set the right margin on even pages, in case you are going to print both sides of the paper.