I make a bit change in your coordinates so that the figure fits better with your document. If it is not good, just change the coordinates in the code.
You can simply add strings to a TikZ picture by a simple \draw command:
\draw (<x>,<y>) node {<string>};
Some option can be added, for example
\draw (<x>,<y>) node[left=0.5cm] {<string>};
For example, this is your diagram:
\documentclass[tikz, border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}
\draw (0,0) -- (6,0);
\draw (2,-0.1) -- (2,0.1);
\draw (4,-0.1) -- (4,0.1);
\draw (2,0) node[below=1ex] {$-3$};
\draw (4,0) node[below=1ex] {$-6$};
\begin{scope}[red] % For local settings -- thanks to @marmot's comment
\draw (1,0) node[above] {$+$};
\draw (3,0) node[above] {$-$};
\draw (5,0) node[above] {$+$};
\end{scope}
\end{tikzpicture}
\end{document}

\drawcommand,\nodeis enough. – AndréC Jan 08 '19 at 04:57\drawcommand is more basic than\node(at least to me), and as the OP shows that he/she doesn't know how to add text, I decide to show her the simplest way. – Jan 08 '19 at 05:02\drawis an alias for the\path[draw].[draw]is an option of the\pathoperation, see section 15 "Actions on Paths" of manual 3.0.1a. – AndréC Jan 08 '19 at 05:56\inf:)) to achieve the desired result (in his free time for zero money without any warranty). – Dr. Manuel Kuehner Jan 08 '19 at 09:21\node. Saying you have to write\draw ..... nodeis not true. When answering a question, you have to be accurate. He gives his personal opinion that there is no support in the manual. – AndréC Jan 08 '19 at 13:27