tell
The German word 'sagen' is used to express the act of communicating information to someone. It is often used in the same way as the English word 'tell'. For example, 'Er sagte mir, dass er krank ist' means 'He told me that he is sick'.
I have to tell you something.
In this sentence, sagen means 'to say' or 'to tell'. It is used here to express the need of the speaker to convey a piece of information to someone else, with 'dir' meaning 'you'.
He cannot say why he is crying.
This phrase also uses 'sagen' to mean 'say'. Here, it's used to show the inability of 'he' (er) expressed by 'kann nicht sagen' to articulate the reason for his tears, implied by 'warum er weint'.
She has nothing to say.
In this context, 'sagen' means 'to say', and is used to express the fact that 'she' ('sie') doesn't have anything to express or communicate, stated by 'hat nichts zu sagen'.