you should know how to access and configure your router using your web browser. Im not going to tell you how to do that.
find your way into the "ports", "virtual servers" or "port forwarding". The services can go by any of these names. If you have a modern router, you should see options for "inbound port" which are any two ranges, "type" which is tcp or udp or both, "private ip address" and "private port".
say you want to open or "foward" ports in your firewall for limewire. Say port 4321 is chosen for limewire. Specify the "inbound port" range as "4321 - 4321". private ip address should be the first 3 numbers of your gateway, which should already be inputted..and the last number is the number of the computer you want to open it up for. check for your pc number in "DHCP table or DHCP client list". For type, choose tcp. Put the same ranges for private ports.
for programs like bittorent, it is wise to configure a smaller range in the program. I use 7000-7010, which lets me have up to 10 sessions at a time.