How do I activate a specific workbook and a specific sheet?

You do not need to activate the sheet (you'll take a huge performance hit for doing so, actually). Since you are declaring an object for the sheet, when you call the method starting with "wb." you are selecting that object. For example, you can jump in between workbooks without activating anything like here:

Sub Test()

Dim wb1 As Excel.Workbook
Set wb1 = Workbooks.Open("C:\Documents and Settings\xxxx\Desktop\test1.xls")
Dim wb2 As Excel.Workbook
Set wb2 = Workbooks.Open("C:\Documents and Settings\xxxx\Desktop\test2.xls")

wb1.Sheets("Sheet1").Cells(1, 1).Value = 24
wb2.Sheets("Sheet1").Cells(1, 1).Value = 24
wb1.Sheets("Sheet1").Cells(2, 1).Value = 54

End Sub

You have to set a reference to the workbook you're opening. Then you can do anything you want with that workbook by using its reference.

Dim wkb As Workbook
Set wkb = Workbooks.Open("Tire.xls") ' open workbook and set reference!

wkb.Sheets("Sheet1").Activate
wkb.Sheets("Sheet1").Cells(2, 1).Value = 123

Could even set a reference to the sheet, which will make life easier later:

Dim wkb As Workbook
Dim sht As Worksheet

Set wkb = Workbooks.Open("Tire.xls")
Set sht = wkb.Sheets("Sheet2")

sht.Activate
sht.Cells(2, 1) = 123

Others have pointed out that .Activate may be superfluous in your case. You don't strictly need to activate a sheet before editing its cells. But, if that's what you want to do, it does no harm to activate -- except for a small hit to performance which should not be noticeable as long as you do it only once or a few times. However, if you activate many times e.g. in a loop, it will slow things down significantly, so activate should be avoided.