I am taking my first timid steps into Apex development and have run up against a problem. Even though all the apex classes being used so far are managed or from the SalesForce App Exchange, I get an error saying I have 23% code coverage when trying to deploy. How can I determine where the fault may lie or run something to improve the situation?

I am taking my first timid steps into Apex development and have run up against a problem. Even though all the apex classes being used so far are managed or from the SalesForce App Exchange, I get an error saying I have 23% code coverage when trying to deploy. How can I determine where the fault may lie or run something to improve the situation?