How do you write northbound?

You could use the word northbound like this: The northbound lanes are closed. In that sentence, northbound is an adjective that describes the lanes. They are northbound lanes, or lanes in which cars move in a northern direction.

Should north and south capitalized?

You should capitalise 'North', 'South', 'East' and 'West' when they're part of a proper noun (i.e. the name of a unique thing). The obvious examples of this are countries, states, cities, and other geographical areas: We study the history of Eastern Europe.