Userform data entry

Hi all, it's been a long time since I've used Excel (pre ribbon), but I've managed to dig a hole for myself and wondered if someone could throw me a rope.

We have a spreadsheet at work that they duplicate each year for recording some 800+ batches of material. The first record on the spreadsheet begins at C3 and goes through to BJ3, to include the many components within each batch. Now, because we're a bit slack in the foundry this week I was asked to jump on board and try to improve the sheet. Like an idiot I suggested that a userform for data entry might be better than tabbing from left to right across the thing, and hence got lumbered with it despite my protests. I've researched here and other sources and I think I'm on the way but would appreciate any guidance anyone can give.

I've created the userform and it opens on a double-click in any empty cell in column C. Basically I need the form to look for the first empty row in column C and then add all the user input from the form into that row. Also, can data transfer be achieved using code such as;

Cells(emptyRow, 3).Value = txtbox3.Value
Cells(emptyRow, 4).Value = txtbox4.Value

Sorry to be a pain,

Steve.