I see a LOT of people screwing this one up, and a couple of times I have too. So I thought it would be helpful to show the differences.
Than is often used as a comparison of two things. Like you're bigger
than that or that building is smaller
than that one.
Then is often used as a description of time or in a sequence of events/objects. Like I want to go home
then go to bed, or add tomatoes, lettuce, cucumber,
then dressing.
Some people like to remember them as "than" being used for comparisons and contrast, and "then" being used in logic and time. There's an
awesome list of examples here to help you out if you're stuck too
.