You don't. If you don't care already, you probably won't start caring. Nor will you force yourself to care, because if you had that kind of willpower you wouldn't have a problem.

However.

You can learn to find things you do care about in those things you don't. For example: I was working on a gaming project of immense stupidity. No one in the team cared about it. But there was a tiny aspect that was appealing to our lead developer. So he grabbed it, clung onto it and threw himself at the project carrying this sacred piece of "I DO CARE" into the fray.

I'm not very good at this technique, but I saw it work like a charm!