If you are interested in the "Living Latin" as a spoken language as Foster teaches, you can't really do that on your own. But really, most people interested in Latin aren't really interested in that but really just want to read Latin. And you certainly can do that on your own. There are lots of textbooks, but probably the most popular is Wheelock's Latin which has the advantage that if you don't understand a particular exercise, you'll have no problem finding someone to explain it to you on the Net.