In general terms you should at your office's router or firewall add a static NAT to map an internet public address to the internal IP address of your web server.
Then you should allow traffic to port 80/443/other on your public IP address to the range of IP address you want to provide this access to (assuming it is not the whole Internet).
Or you could have your clients (and/or you) connect via VPN (IPSec/SSL/L2TP) to your office and then to your server.
Not knowing what kind of firewall/router and network topology you have makes it difficult to give more detailed instructions.
Anyways, regardless of the kind of hardware you have, it is not a good security practice to allow access from outside your network to resources on your internal network. There maybe misconfiguration/bugs on your web server or on the applications that are running on top of it and this may jeopardize the security of your server and by cause and effect, of your internal network. This is what DMZs are good for.