Then fill in the dialog box for the number of Rows and Columns you want. Don't worry, you can add more later. You can set the width and height of the cells, or let the system adjust them automatically.
If you click in a cell, you can type in it. You will also see pull tabs that allow the dimensions of the table to be changed.
You will also see arrows and circles with an "X." If you hover your mouse over these symbols, they will be highlighted. Clicking a horizontal arrow will cause a new column to be created to the left or right of the column you are in, and clicking a vertical arrow will cause a new row to be created above or below the row you are in [the vertical arrows are hard to see in the left-most cells of a table]. And clicking one of the X circles will cause that row or column to be deleted. Try it: you can always reverse your actions by hitting CTRL-Z. Highlighting one or more rows or columns and then hitting the delete key will delete the data in that row or column (not the row or column itself).
For more complicated options, right click in any cell.
Tables can also be imported from a word processing program: copy the table and then paste it into the LocalWiki page with CTRL-V.