I know that להתכונן means to prepare oneself for an action, such as getting ready for a race. Can it also be used in "prepare (yourself), this is not going to be nice"?. I suppose you could also say in English 'be prepared'. Is להתכונן, להכין or something else used?

 

You can say lehitkonen for something bad.
The difference between lehachin and lehitkonen is that the first is active, second reflexive.

 

תתכונן לצרות sounds well.
תכין את עצמך לצרות is also good, less frequent I guess.
תהיה מוכן לצרות works too.

 

Thanks very much.