Create a table in richtextbox

You can do it by directly setting the RTF. Here's a sample that should give you some idea what you want to do:

{\rtf1\fbidis\ansi\ansicpg1252\deff0\deflang1033{\fonttbl{\f0\fswiss\fprq2\fcharset0 Calibri;}{\f1\fnil\fcharset0 Microsoft Arial;}}
{\colortbl ;\red0\green0\blue0;}
\viewkind4\uc1\trowd\trgaph108\trleft-108\trbrdrt\brdrs\brdrw10\brdrcf1 \trbrdrl\brdrs\brdrw10\brdrcf1 \trbrdrb\brdrs\brdrw10\brdrcf1 \trbrdrr\brdrs\brdrw10\brdrcf1 \clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx1576\clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx3260\clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx4944\pard\intbl\ltrpar\sl276\slmult1\f0\fs22 TopLeft\Top Right\cell\row
\trowd\trgaph108\trleft-108\trbrdrt\brdrs\brdrw10\brdrcf1 \trbrdrl\brdrs\brdrw10\brdrcf1 \trbrdrb\brdrs\brdrw10\brdrcf1 \trbrdrr\brdrs\brdrw10\brdrcf1 \clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx1576\clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx3260\clbrdrt\brdrw15\brdrs\clbrdrl\brdrw15\brdrs\clbrdrb\brdrw15\brdrs\clbrdrr\brdrw15\brdrs \cellx4944\intbl Bottom Left\cell Bottom Right\cell\row
\pard\ltrpar\sa200\sl276\slmult1\par
\pard\ltrpar\f1\fs17\par
}

You should really read up on the RTF format if you want to do this. Alternatively (and here's a handy hint), create the page you want in Word, and save it out as an RTF file. You can use this as the basis of creating your application.