You need a rewriteable game cartrige. Such a cartrige is called a flashcard. From the data storage point of view, there are two types of flashcards: with built-in memory, or memory card adapters.
The ones with the built-in memory are simple: just plug them in the reader that you recieve with them, plug the reader in the USB on your PC, and transfer the ROMs; then plug the flashcart into your GBA and play.
With the adapter type ones you need a flash-based memory card to store ROMs. Such memory cards are used in digital cameras, some mp3 players, and some mobile phones. The 8-in-1 reader on your laptop can read and write to those memory cards. You then insert the memory cards into the adapter flashcard, plug it into the GBA, and play.
The flashcard you have to order via Internet.
If you don't have a credit card, you're stuck, unless your parents will order a flashcart for you.
For details, reccomendations, reviews, critiques and the like, read the "flashcart reviews", and the rest of the forum.
This has been a public service announcement.
Thank you.