documentclass[12pt,%
               %11pt,%
               %10pt,%
               ]{article}
usepackage{fancyhdr}
pagestyle{fancy}

usepackage{showframe}

begin{document}
%vspace*{dimexpr 2.5pt-baselineskiprelax} % correct first baseline in body positiong for 12pt
%vspace*{dimexpr 1.6pt-baselineskiprelax} % correct first baseline in body positiong for 11pt
Test line 1st page
newpage

Test line 2nd page
end{document}