Earlier in this section, Morrie is talking about how he believes in the inherent good of people, but admits that society is what destroys them. He argues that humans become "mean" when they are threatened, and gives the example of how money drives our culture. When it comes to making money and keeping it, and the possibility of losing it, he basically explains that people only look out for themselves.
The "subculture" he advises Mitch to build for himself then, is one that obeys society's basic rules, but does not allow society to deem what is important for individuals. Essentially, he's telling Mitch to create his own values or his own set of things which are important to him. He specifically advises him against letting what is popular among everyone else be the determining factor for what is important. He then uses himself as an example of someone who has been able to do just that.
Here's what I mean by building your own little subculture," Morrie said. "I don't mean you disregard every rule of your community. I don't go around naked, for example. I don't run through red lights. The little things, I can obey. But the big things- how we think, what we value-those you must choose yourself. You can't let anyone-or any society-determine those for you. (155)