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One of the more difficult concepts to grasp about Excel VSTO programming is that you don't refer to cells like an array, Worksheet[0][0] won't give you cell A1, it will error out on you. Even when you type into A1 when Excel is open, you are actually entering data into Range A1. Therefore you refer to cells as Named Ranges. Here's an example:
Excel.Worksheet sheet = workbook.Sheets["Sheet1"] as Excel.Worksheet; 
Excel.Range range = sheet.get_Range("A1", Missing.Value)

You can now literally type:
range.Text // this will give you the text the user sees
range.Value2 // this will give you the actual value stored by Excel (without rounding)

If you want to do something like this:
Excel.Range range = sheet.get_Range("A1:A5", Missing.Value)

if (range1 != null)
     foreach (Excel.Range r in range1)
     {
         string user = r.Text
         string value = r.Value2

     }
 

public static Excel.Workbook OpenBook(Excel.Application excelInstance, string fileName, bool readOnly, bool editable,
        bool updateLinks) {
        Excel.Workbook book = excelInstance.Workbooks.Open(
            fileName, updateLinks, readOnly,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing,
            Type.Missing, editable, Type.Missing, Type.Missing, Type.Missing,
            Type.Missing, Type.Missing);
        return book;
    }

public static void ReleaseRCM(object o) {
        try {
            System.Runtime.InteropServices.Marshal.ReleaseComObject(o);
        } catch {
        } finally {
            o = null;
        }
    }

using System;
using System.IO;
using System.Reflection;
using NUnit.Framework;
using ExcelTools = Ms.Office;
using Excel = Microsoft.Office.Interop.Excel;

namespace Tests
{
    [TestFixture]
    public class ExcelSingle
    {
        [Test]
        public void ProcessWorkbook()
        {
            string file = @"C:\Users\Chris\Desktop\TestSheet.xls";
            Console.WriteLine(file);

            Excel.Application excel = null;
            Excel.Workbook wkb = null;

            try
            {
                excel = new Excel.Application();

                wkb = ExcelTools.OfficeUtil.OpenBook(excel, file);

                Excel.Worksheet sheet = wkb.Sheets["Data"] as Excel.Worksheet;

                Excel.Range range = null;

                if (sheet != null)
                    range = sheet.get_Range("A1", Missing.Value);

                string A1 = String.Empty;

                if( range != null )
                    A1 = range.Text.ToString();

                Console.WriteLine("A1 value: {0}", A1);

            }
            catch(Exception ex)
            {
                //if you need to handle stuff
                Console.WriteLine(ex.Message);
            }
            finally
            {
                if (wkb != null)
                    ExcelTools.OfficeUtil.ReleaseRCM(wkb);

                if (excel != null)
                    ExcelTools.OfficeUtil.ReleaseRCM(excel);
            }
        }
    }
}