shortest way to texting in Lollipop
Using absolutely stock, as-installed-by-default stuff, the fastest way would be:


Swipe to unlock
Tap home button
Tap Messenger or Hangouts (which you have put as an icon on your home screen)
Tap new message.


Possible ways to reduce the number of steps: 


Third-party SMS apps which provide homescreen widgets that take you straight into composing
Third-party lock screen replacements
Alternative ROMs with different lock screen arrangements (e.g. some Cyanogenmod versions let you define different apps to be launched depending on the direction of your unlock swipe)