Try this:
/setblock ~ ~1 ~ wall_sign 5 [replace|destroy|(none)] {Text1:"{\"text\":\"Your Text Here\"}"} << never forget to use backslashes before an opening or closing quotation.
If you want it to have colors:
/setblock ~ ~1 ~ wall_sign 5 {Text1:"{\"text\":\"Your Text Here\",\"color\":\"your_color\"}"} << on using colors, for spacing (such as red or dark red), use an underscore to indicate it as space ; see colors here: http://minecraft.tools/en/color-code.php , use the color names; never forget to use backslashes before an opening or closing quotations, right :) .
For text styles:
/setblock ~ ~1 ~ wall_sign 5 {Text1:"{\"text\":\"Your Text Here\",\"color\":\"your_color\",\"bold\":\"true\"}"} << existing style tags: bold, italic, underline, obfuscated (or randomizing text) only use boolean responses on these tags (like true or false. 0 or 1 is not accepted.);
To put text on appropriate text line: For 1st line: Text1 tag; For 2nd line: Text2 tag; For 3rd line: Text3 tag; For 4th line: Text4 tag; (take note that these Text~ tags have same tags inside, like the colors and style tags); (also take note that signs only have 4 lines :) .)
Want all lines to be put? Use them altogether.