I just wanna download a app

Unfortunately you have not provided sufficient information for us to propose a solution. Describing what happens when you try, and the exact wording of any error messages, would greatly aid us in helping you. In the meantime see these documents if you think they apply:

Click on this link "If you can’t download or update apps on your iPhone or other Apple device" - https://support.apple.com/HT207165

If the first 5 don't help, use the 6th.

If instead you see a message that says "Your account has been disabled in the App Store and iTunes" click on this --> https://support.apple.com/HT208856