Change event procedure

I am struggling with Excel 2010 VBA. I have some checklists that various people will provide input to. I want to:
1. enter text into a box (or activate a macro to do that, such as Ctr d ="Done", or Ctr n = "N/A") and then 2. have the cursor move to the cell in the next column and type the Application.UserName and then
3. have the cursor move to the cell in the next column and type the current date.

I can do steps 2 & 3 but can't work out how to do Step 1 automatically. I understand that it is something that responds to a change in the worksheet/cell, Also I need to make sure that when a different user accesses the worksheet the UserName does not change.