Assuming your server is on a private IP address inside your network and the ASA is the path to/from the Internet, you need to:
a. create a static NAT for your server's private address so that it will have a publicly addressable address on the ASA.
b. Create an access-list to allow incoming traffic on the necessary ports. Those are:
PPTP IP Protocol=TCP, TCP Port number=1723
L2TP IP Protocol Type=UDP, UDP Port Number=1701
GRE IP Protocol Type=TCP, UDP Port Number=47
You can do these two thing together most easily via Configuration > Firewall > Public Servers. For the "Public Services" create a new named service group that includes the above TCP and UDP ports.