I have a 6 month old, haven't had my period since her birth. Can I get pregnant? If so how do I calculate my safe period?

YES. You can get pregant if not using some type of protection. Can not really predict "safe period" without having regular cycles, and that is not an absolute guarentee.
You can. You will ovulate 2 weeks before you have your period, so it is definitely possible to get pregnant before your period returns. If you don't want to use methods of birth control other than natural family planning, a good reference is "taking charge of your fertility" by toni weschler. You can also see her website, tcoyf.Com.