I have a problem for downloading
How can I installing application?
If i understand your question you are trying to download apps from the App Store?
Did you already look at Settings > General > 'Limitations' > type in your password > look at 'Install Apps' which is maybe turned off. If you turn that you should able to download apps again from the App Store as long as you know your password for your account so you can buy or download free apps. In all cases it will ask for your password if you download a new app.