When you delete your files, your information isn’t truly gone. Deleting a file on your device simply means it's been moved to an area where you can no longer access it.
The reason this is the default action on your computer is that it takes much less time to delete a file than to permanently remove it. So you get a higher level of usability and a greater chance of retrieving files that you delete accidentally. The downside is that your information could be accessed with specialized software.
To permanently remove your files, you'll need to use a different process, which varies depending on what type of device and operating system (OS) you're using. For a Windows PC, you can run a program like Secure Eraser or CCleaner to permanently remove your files. They work by overwriting information in such a way that it can never be retrieved, even with specialized software.
If your laptop is a Mac, read about the steps you need to take to permanently remove your data. Don't forget Step #2, which is to deauthorize your computer from using iTunes; this will prevent another possible negative outcome, which is others being able to make purchases from your iTunes account.
The process is similar if you want to wipe all data from a smartphone or tablet; search online for information about your particular manufacturer or OS.