top=10mm, bottom=10mm, left=10mm, right=10mm            // or
tmargin=10mm, bmargin=10mm, lmargin=10mm, rmargin=10mm  // or
margin=10mm

etc.

documentclass[
    12pt,
    a4paper,
    ngerman
]{minimal}
% i use minimal because i don't need header, footer, page number etc.
% i just need a simple textbox

usepackage{lipsum}
usepackage{showframe}

usepackage[
    top=10mm,
    bottom=10mm,
    left=10mm,
    right=10mm
]{geometry}

begin{document}

lipsum{1-8}

end{document}

documentclass[
    12pt,
    a4paper,
    ngerman
]{minimal}
% i use minimal because i don't need header, footer, page number etc.
% i just need a simple textbox

usepackage{lipsum}
usepackage{showframe}

begin{document}

lipsum{1-8}

end{document}