I found a similar solution to this question in c# How to Select all the cells in a worksheet in Excel.Range object of c#?
What is the process to do this in VBA?
You can simply use cells.select to select all cells in the worksheet. You can get a valid address by saying Range(Cells.Address).
If you want to find the last Used Range where you have made some formatting change or entered a value in you can call ActiveSheet.UsedRange and select it from there. Hope that helps