How do I set a password for purchase new apps

to set password for new app purchases:

On your iPhone, iPad, or iPod touch

1. Open the Settings app.
2. Tap [your name], then tap iTunes & App Store.
3. Tap Password Settings. ... 
4. Under Purchases and In-App Purchases, tap the setting that you want.
5. Under Free Downloads, tap Require Password to turn the setting on or off.
6. When asked, enter your password.

Manage iTunes and App Store Password Preferences