Here's a quick tip to get started working with Excel interop in C#.

// Create a new instance of the Excel application
excelFile = new Excel.ApplicationClass();
Excel.Workbook workbook = excelFile.Workbooks.Add(Type.Missing); // Now create a brand new workbook
excelFile.Visible = true; // ensure that the excel app is visible.
Worksheet ws = (Worksheet)excelFile.ActiveSheet; // Get the current active worksheet.
Microsoft.Office.Interop.Excel.Worksheet worksheet2 = (Microsoft.Office.Interop.Excel.Worksheet)workbook.Worksheets.get_Item(2); //Get more work sheet if neccessary

ws.Activate();

//Here is how to set the column Width

public void SetColumnWidth(Worksheet ws, int col, int width)
{
 ((Range)ws.Cells[1, col]).EntireColumn.ColumnWidth = width;
}

// Apply the setting so that it would autofit to contents
public void AutoFitColumn(Worksheet ws, int col)
{
 ((Range)ws.Cells[1, col]).EntireColumn.AutoFit();
}

How to Set Row to Bold in Excel C# Interop:

((Range)ws.Cells[row, 1]).EntireRow.Font.Bold = true;

How to Set a value inside a cell in Excel:

(Range)ws.Cells[row, column]).Value2 = item; // set the cell value at a row and column

How to Format the Entire Column:

((Range)ws.Cells[1, col]).EntireColumn.NumberFormat = format;