Extending Access List

I kind of get confused with applying access list, either as an in or out on interfaces. Is there someone that can explain it really well
Trying to add access-list 101 permit tcp any x.x.x.x eq 80 to fa0/0 on our LAN? The aim is to allow any one from the internet to get through to the server ( x.x.x.x) only via port 80.
So do i apply as

ip access-group 101 in or ip access-group in

Thanks