i found that my father is in relationship with a girl....i seen some msg on my dad's phn....what to do suggest me plz????

I don't know what is your age and how your father see you but for your father you are a sincere and responsible son then you can actually ask him directly about this and tell him that you have told mother anything about it, he will actually answer you knowing that you may tell this to mother.
And also don't tell your mother at all and also I suggest you to look for more proof for your father's relation before asking him out.

Well this is shocking for a boy to know something like father having relationship with some girl outside home.
coming to your point, yes, you are a responsible child of your parent so I think you should directly ask your dad about this, if he denies you may threaten him to confess his guilt if not you may approach your mother with the truth, but keep all this for the later stage for now, collect some more evidences of this fact and then with all those support n evidences, approach your father so that he can't deny on what is the truth...

i m 20 years old..........how i can got some evidence on this case...my father not give me his phn....they put it always with him.....can i have some supporter for it????

my age is 20.....how i can got proof my dad not give his phnto other....nd also delete all msg....can i hv some supporter for that??

how i can got evidence.....dad don't give his phn also delete all msg???

Keep a spy on him as to where he goes at what time and accidently check his phone while he is bathing because thats the only time may get his fone away from him....keep spying till u get the exact details pf his doings in a day n den catch him red handed after keeping a good watch on him....yes u can have some supporters for u to help you out