copying same formula with only one cell change

i'm trying to get a simple formula A1*C2 (C2 being the non changing cell) to follow down loads of columns.
eg. A2*C2, A3*C2, A4*C2 etc etc
by clicking the right bottom corner and drag down, it increases the value of both cells involved.
can someone help? (i'm sure this is an easy one...)

You can use $ to fix either the row reference or the column reference, e.g. in your case to prevent the row reference (the 2) in C2 changing as you copy down, change it to C$2, i.e.

=A2*C$2

If you want to copy the formula across without the column reference (the C) changing then put $ in front of the C, i.e.

=A2*$C$2

thank you ))