If you have a router, it should be explained in its user manual how to forward ports. It usually involves specifying a port range or specific ports, and which computer they are to be forwarded to. If you have a router that supports UPnP, you could make sure that feature is enabled, and then enable UPnP in your BitTorrent client. That would make the BT client attempt to automatically configure the port forwarding.

Regardless of the speed of your internet connection, having a port forwarded for BT will always increase your average download and upload speeds, because you are able to connect to a larger number of peers and seeds, thus increasing the chance of finding one who has a great upload speed.

If you're using Azureus, a yellow, ":|" smiley signifies that you can download, but do not have a port forwarded, meaning you aren't downloading as efficient as you could be doing. I've been stuck with yellow smileys for 3 years now, it's annoying the hell out of me.

If you're going to turn on encryption to prevent "traffic shaping", you should enable fallback options, else you would probably dramatically reduce the amount of potential seeds/peers.