Sounds like an interesting project but I'm not sure if a 1000 internetz and Google's favours are a reward or a curse. Nevertheless, embarking on your first VBA project can be daunting and there are several people in this forum who are expert at it but you will need to provide very detailed information about the format of all the sheets involved. But I'll start off with some general advice on one (of very many) ways to proceed.
This is a bare bones macro that will get your first unused PO# assuming that the Purchase date cell is in column B and is empty;
It will copy the PO# from Sheet2 (substitute your PO sheet name) and paste it into Cell A5 in Sheet1 (substitute your target cell and Invoice worksheet name)
You can then either use similar code to copy the target cells from your Invoice sheet and place them in the PO sheet or you can use normal excel functions like vlookup or index to to get the values from the Invoice to the PO sheet using the PO# as the reference.
Give this some thought and come back with any questions at all that you may have. We are always happy to help a budding Excel fanatic.