Janice -
The formula Alan gave you should work. What the formula is saying is IF the Vlookup does not find a match (thats what ISNA means), then bring back a "no" otherwise bring back a "yes". The Vlookup statement itself is broken down as follows:
A1 represents the cell where your ID'd start in sheet 1

Sheet2!A:A represents where you want to search for a match (in this case, on sheet 2 in all of column A)

1 means bring back the 1st column in your range (which in this case is only column A, so you only have 1 column)

false means you want an exact match

I hope this helps. Good luck!