One of my relatives told me do not listen to unknown mantra showing on you-tube since do not know the actual meaning of the mantra and not surely correct pronunciation. How to identify the real good mantra from bad one ? can I chat the mantra appear in the google search engine? is there any bookshop to buy the mantra book(good) and If I chat with the wrong pronunciation what will happen? if it is bad then is there any organization available to certify the good mantra?
Yes, mantras when pronounced incorrectly can have negative effects. This chapter of the Taittiriya Samhita of the Yajur Veda gives an example of this
Tvastr, his son slain, offered Soma excluding Indra. Indra desired an invitation to the rite, but he did not invite him, (saying), 'Thou hast slain my son.' He made a disturbance of the sacrifice, and forcibly drank the Soma. The remains of it Tvastr cast upon the Ahavaniya (fire), saying, 'Hail! wax great, Indra's foe.' While (the fire) was flaming upwards to strike, just then of itself it stayed.
The problem is that Tvashta mispronounced "Hail! wax great, Indra's foe," so instead of strengthening Vritrasura it ended up strengthening Indra.
To answer your other question, you shouldn't learn mantras from YouTube videos or books, but from a guru belonging to a recognized Parampara. Here is what chapter 10 of the Garga Samhita says:
Vamana, Brahma, Ananta Sesha and Sanaka Kumara will appear as brahmanas by the order of Visnu, for the preservation of eternal righteousness in kali yuga. Visnu Swami, Madhvacarya, Ramanuja and Nimbaditya will appear respectively as a portion of Vamana, Brahma, Ananta Sesha and Sanaka Kumara. These saviours will be the establishers of the four authorised and empowered spiritual channels of disciplic succession in the period calculated from the reign of King Vikrama in 54 B.C. subsequently through the present era of kali yuga. These four authorised and empowered spiritual channels of disciplic succession are to be fully accepted by all beings; as any word, combination of words or formulation of sounds frequencies, invoked or addressed, audible or inaudible, secret or revealed, ancient or contemporary, outside their auspices prove to have absolutely no efficacy.