What kind of "CERTAIN DATA"? Can it be anything other than ""?
If you type in this "CERTAIN DATA" what's in cell A1 sheet2? Is there more
or is the only data in just cell A1 in sheet2?
If you want to do a lookup, then you can start by assigning a named range
for your lookup data. Then use data validation in the appropriate cell (ie
a4). In cell a3 you would have to write a vlookup or index match or whatever
based on the value in cell A4.
=vlookup(a4,yournamedrange,number of columns to the right that you want a
new value to appear, # of rows if applicable)