You don't HAVE to open your ports but it helps your speeds tremendously. I would Google "open ports for xxxx modem" and you'll probably find it. Depending on what client you use, you can open one or a range of ports. For everything to work properly you need to have the same port(s) opened on your client, firewall, and modem. The modem is what is usually the stickler for people.

If you're just starting out with BT I suggest reading as much as you can about it. The FAQs here and at bt.etree.org are good places to start. Much of it won't make a lot of sense at the start but you'll start to figure it out eventually. If you have any specific questions, this forum is the place to come!