This is, first of all, a promise - that God will be with Moses, regardless of what happens. Then, God also states that, when he has brought the people to this place, he will worship God upon this mountain.
The Pulpit Commentary says,
Certainly I will be with thee. Literally, "Since I will be with thee." Moses had excused himself on the ground of unfitness. God replies - "Thou wilt not be unfit, since I will be with thee - I will supply thy deficiencies - I will impart all the qualities thou needest - and this shall be a sign unto thee of my power and faithfulness - this shall assure thee that I am not sending thee upon a fruitless errand - it is determined in my counsels that not only shalt thou succeed, and lead the people out, but after that, - when thou hast so done - thou and they together shall serve me on this mountain."