It is common to use a ListView instead of a RichText box to show tabular data. So most examples that can be found will be for ListViews (or even recordsets). They will show you how to get the Excel data for each cell (line by line and row by row).
If you still want to use a RichText box and want to dispaly the data as simple text, just append the content for each cell using appropriate separators. If you need a formatted table, you have to read the RTF specification regarding tables (which is rather complicated).
There are also different methods to read Excel files:
COM interop which requires an installed Excel version, and Excel database drivers (OleDb) or 3rd party DLLs which both do not require Excel to be installed.
Some links from CP about reading Excel with C#:
Working with Excel Using C#[
^] (Interop)
C# How To Read .xlsx Excel File With 3 Lines of Code[
^] (DLL)
Read Excel File into DataSet in ASP.NET Using C#[
^] (OleDb)