If you can assume that each row in column x has data, such that the first empty one is the one you want to write in, you can loop through the cells in that column (I think that would be cells(rowNumber,colNumber), but I can't check it right now) looking for the empty one.

However, this is best if it only happens once. If you will be writing many times, it would be best to keep the next row to write to (or last one written to) as a variable in the program. Figure it out once, then manage it in code.