trying to write a loop

Hi, I'm very new to writing macros in Excel, in fact I usually just record something and then go in and clean it up a little when I need a macro. But I have this frustratingly simple thing, where I have values in adjacent cells (cols B and C) that need to be in the same column (B, one under the other).

So I need a little macro that will insert blank rows in between the existing rows and then one that will move the value from column C to one line number greater in column B.

I just recorded a few steps of it as a macro, and it worked fine for small sets of data, but now my boss has told me we will be getting datasets of 3,000 lines. My script will be gigantic unless I can figure out how to make Excel repeat itself down column A until it finds a blank cell.

Please help? Thank you so much.

--Susan