Select the cell then click on Data | Validation and on the pull-down
for Allow select List. If you just have a simple restricted list like
A, B, C, D then you can enter this directly into the Source box with
each item separated by a comma. If your list is more complex, then in
the Source box you should put a range that refers to where your list
is, e.g.:
=$D$1:$D$10
if you have a list of acceptable items in the cells D1

10 of the same
worksheet.
If your list is on another worksheet you will need to give the range a
name, eg "source_data", and then you can enter this named range in the
Source box.
Click OK when you are finished.
Hope this helps.
Pete