Hi Hannah.
The best way to protect the xNT is with the Dangerous Things app (on Android). It lets you set a password and it sets the lock bits to prevent anything messing up the tag.
For writing, I like NFC Tools by wakdev, (also on Android). The Pro version is worth the money. It reads and writes, lets you write multiple NDEF records to the tag, etc. and has some other tools and functions.
Other good ones (on Android) are NFC TagInfo by NXP is good and thorough, and NFC Tag Writer also by NXP, has some good functions.
For iPhone, the options there are all read-only due to Apple restrictions, but NFC TagInfo by NXP is a good app for reading tags.
Good luck.