I heard in the name of Rabbi Chaim Shmuelevitz that if you believe in a person his Blessing will come true. Does that mean even if the person is simple he is not so big in Torah, if you believe in him that his Blessings will help him with his problem, it will it work?
I am not familiar with the paragraph you refer to in the name of Rav Shmuelevitz, but it sounds like it’s been misquoted. Judaism believes in God and not in human beings, great as they may be. There is a source in the Talmud that one can and should ask great tzadikim to (also!) pray to God for for his aid (Sukka 14a), and that even the prayer of a simple person shouldn’t be taken lightly (Megilla 15a). Nevertheless, we serve Hashem, and He doesn't serve anyone! Only He decides when the answer is "yes". The main point is for the person in need, who will obviously have special kavana (concentration) because he really needs it, to pray directly to Hashem. In fact, this is a basic difference between Judaism and Christianity, that we don't need a "go-between", and speak directly with our Loving Father.