Without more detail - look up data validation - this will solve your answer.  

if you don't know where this is on the menu bar you should see

File .. edit.. .. .. . Data.

Select the data drop down and then the validation..

The next question may be do you want to key in the list

You might want to google Excel Data Validation to get other examples....

How many different selections will there be from the drop down? If there's
only a "few" (3, 4 or 5) you could do something like this:

=IF(D3="Smith",200,IF(D3="Jones",125,IF(D3="Adams",750,"")))

If you have many selections then you should build a lookup table:

http://contextures.com/xlFunctions02.html