How do I use only fingerprint to unlock my phone?
That isn't possible.

Android encrypts a lot of your data when the phone turns off so no one can access your files. The key to that encryption, however, is the hard password you don't want to use. A fingerprint cannot be used a an encryption key because it is a system that has an error tolerance, and therefore does not work as an encryption key. 

In other words, this is impossible under any circumstance. It's just how Android works.