Apart from the fact that they look and sound similar, they have no similarities. "Then" is used to describe what happened next in a sequence of actions. "Than" is used in comparisons. Eg. "First I went to the beach, then I went to the chemist." "Today is colder than yesterday."

Then- is used to describe that comes after Than- is used to describe the difference Example: I took a shower and 'then' went to bed. I would rather drive my car 'than' ride the bus.