Begin with downloading VSTO (Visual Studio Tools for Office) this will allow you to created a C# Excel Add-In.
In Visual Studio when you create a new project you will see Office and you will be able to select Excel from it.
Start from there, once you do that you can come back and ask more specific questions.
Some useful tips with working with Excel.
To Select an Active Sheet:
Excel.Worksheet sheet = this.Application.ActiveSheet as Excel.Worksheet;
To Select a specific range (A1 - B5 in this case):
Excel.Range range = sheet.get_Range("A1", "B5") as Excel.Range;
To Set a value to the whole range:
range.Value2 = 2; //will set every cell in A1 through B5 to 2
You can get values from range in a 2 dimensional array, for instance:
object[,] values = range.Value2 as object[,];
//this will return an multidimensional array representing rows and cols
//as you see in the range. in this case the array is filed with "2"
You can then change the values in the whole array and apply back to the range:
values[2, 2] = 4; //will change the value to 4 in row 2, col 2 of the *range*
range.Value2 = values; //set back the whole range to the array
You can use this technique to update the whole range at once, by preparing an array first and then setting it to the range values.
To get a value from a specific cell in your range (To set a value is the same way but reverse).
Excel.Range cell = range.Cells[1,1] as Excel.Range; //this will take the cell from row 1, cell 1. if you used array this would be values[1,1]
string value = (cell.Value2 ?? "").ToString();
This should allow you to do basic tasks in Excel, you can set values and get values and select ranges. Once you have more specific question, please come back.
Don't forget, arrays coming from Excel are 1-based not zero-based !!!