To edit a table you already have in LaTeX, paste its code in File / From LaTeX code.
To write the code yourself, see our LaTeX tables cheat sheet: examples of lines, merged cells, column widths, colours, captions and long tables, each with the table that LaTeX makes from it.
For more professional looking tables, try the booktabs style (Style: Booktabs in the options under the code). Its tables look like those in books and scientific articles:
The table's horizontal borders become the rules: the top border
\toprule, the bottom one \bottomrule and the
others \midrule (\cmidrule under some columns
only). So give the table a top and a bottom border, e.g. with
Table / Add horizontal borders.
A column is as wide as its longest text. To wrap its text, give the column a
width: drag the right edge of its header above the table, or right-click the
header and choose Set column width (double-click the edge to
make the column as wide as its text again). It becomes a p{}
column of that width. To make the table as wide as the text on the page,
choose Width: Wrap to text width in the options: the
columns with a width share what the other columns leave of it (a
tabularx table). Scaling the table to the text width shrinks its
text too, which most style guides advise against.
Characters with a special meaning in LaTeX, such as %,
&, _, # or $, are
escaped, so the cells print as typed. To write LaTeX in the cells, e.g. math
($x^2$) or \cite{key}, uncheck
Escape special characters in the options.