Main text goes over header

Question

How could I fix it?

Here is the code:

\documentclass[a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[top=1cm,left=1.5cm,bottom=2cm,right=1.5cm,headsep=0.3cm]{geometry} 
\usepackage{tikz}
\usepackage{graphicx}
\usepackage{multicol} 
\usepackage{amsfonts,amssymb,amsmath}
\usepackage{xcolor}
\usepackage{fancyhdr} 
\pagestyle{fancy} 
\usepackage{pbox}
\usepackage{lipsum}

\renewcommand{\headrulewidth}{1.2pt}% linha no header 
\renewcommand{\footrulewidth}{1.2pt} % linha no footer

 \lfoot{\tt Prof. Lucas}
 \cfoot{\LaTeX}
 \rfoot{\tt \today}


\fancypagestyle{firststyle}
{
\lhead{
\def\sc{0.6}
\def\rc{\sc*0.1cm} % rc = 10% de sc
\begin{tikzpicture}[scale=\sc]
\definecolor{tempcolor}{RGB}{50,160,65}
\filldraw[tempcolor][rounded corners=\rc] (0cm,0cm) rectangle (1.2cm,1cm);
\end{tikzpicture}
}   
\rhead{\tikz \node at (0,0) { \textbf{2ª Prova de Física - 2º Semestre}};\\ 
\tikz[baseline]{\node[anchor=base, draw=black, thick, minimum width=8.5cm, minimum height=7mm,rounded corners, yshift=-2mm,label={[fill=white,label distance=-2mm,inner xsep=2pt]90:{\tt Aluno(a)}:}] { \phantom{\tt 15,0} };}   
\tikz[baseline]{\node[anchor=base, draw=black, thick, minimum width=2cm, minimum height=7mm,rounded corners, yshift=-2mm,label={[fill=white,label distance=-2mm,inner xsep=2pt]90:{\tt Turma}:}] {\phantom{\tt 15,0}};}   
\tikz[baseline]{\node[anchor=base, draw=black, thick, minimum width=1.5cm, minimum height=7mm,rounded corners, yshift=-2mm,label={[fill=white,label distance=-2mm,inner xsep=2pt]90:{\tt Valor}:}] { \tt 15,0};} 
\tikz[baseline]{\node[anchor=base, draw=black, thick, minimum width=1.5cm, minimum height=7mm,rounded corners, yshift=-2mm,label={[fill=white,label distance=-2mm,inner xsep=2pt]90:{\tt Nota}:}] {\phantom{\tt 15,0}};}
}
 \lfoot{\tt Prof. Lucas}
 \cfoot{\LaTeX}
 \rfoot{\tt \today}
}

\setlength{\headsep}{\dimexpr\headsep+1.2\baselineskip+.4pt}

\begin{document}

\thispagestyle{firststyle}

\begin{multicols}{2} 

\lipsum[3-20]

\end{multicols}

\end{document}

Answer 1

use

 \usepackage[top=5mm,includehead,headheight=45pt,
             left=1.5cm,bottom=2cm,right=1.5cm,headsep=0.3cm]{geometry} 

Answer 2

Your document produces the warning

Package Fancyhdr Warning: \headheight is too small (12.0pt): 
 Make it at least 44.28043pt.
 We now make it that large for the rest of the document.
 This may cause the page layout to be inconsistent, however.

so adding

\setlength\headheight{45pt}

to the preamble would make things look better.