how can i copy apps from one computer to another?

You don't mention which operating system you have so it's difficult to provide an answer since the mac and Windows stores the files differently.

The other thing you can do is just re-download the apps from the iTunes Store. Just login to the iTunes Store on the other computer using the same Apple ID. On the right-hand side of the iTunes Store page you will see a 'Purchased' link. Clicking on that will take you to a page that will list your past purchases which you can re-download to the new computer.