Before you can insert a table you need to install a plugin called: MCE Table Buttons.
If you do not yet know how to install a plugin, here is an article explaining how.
Once you have done that, go to your "Posts" section and open the article that you wish to insert a table into.
The highlighted button in the image above opens another toolbar row. This is the table button that it will reveal:
When you click on it, a pop up will open.
Input the number of columns and rows you want your table to have.
This is what it will look like:
All you have to do is click inside the cells and type away.