When Hashem asked Moshe to go to Paroh and redeem the Jewish people, Moshe said to Hashem, "Who am I, that I should go to Paroh and take Bnai Yisrael out of Mitzrayim" (שמות ג:י"א). Hashem then responded that He would be with Moshe and He gave him a signal that He indeed was sending him, telling him that when he brings the people out of Egypt, they should serve Hashem upon this mountain (שם פסוק י"ב). What does Hashem's response about Bnai Yisrael serving Him on this mountain have to do with Moshe's question about his being unfit to go to Paroh and redeem the people?
One may suggest that we see here that Moshe was a very humble man, and thus did not consider himself worthy of being the deliverer of the Jewish people. He therefore asked Hashem, "Who am I to do all this?" Hashem then answered Moshe by saying that this very question, the very fact that Moshe asked who he was to deserve to do all this, explains why Hashem chose him to perform this task. This response is hinted at by the reference to serving Hashem on this mountain, which, of course, was Har Sinai.
The Sefer מעיינה של תורה quotes an opinion which says that Hashem prefers to call on those who do not think highly of themselves. For example, out of all the mountains, Hashem chose the lowly Har Sinai, the least impressive of all, as the place on which to give the Torah to Bnai Yisrael. The Midrash tells us that while the higher mountains boasted of their height and beauty, claiming that they were the most worthy of being the place where the Torah would be given, Har Sinai remained insignificant in its own eyes, and was thus ultimately chosen. For this reason, Hashem told Moshe that when he brings the people out of Egypt, they should serve Him upon this mountain. The fact that the Torah will be given specifically on this small and lowly mountain, rather than on any other mighty or tall mountain, such as Har Chermon or Har Tabor, should serve as a sign for Moshe as well, showing that he too is being chosen because of his modesty and humility. Hashem thus explained that like this mountain, Moshe was humble and thus worthy of being the deliverer of the Jewish people.