Determine what makes Jonas feel "sure" of his assignment.

Jonas is worried at the ceremony of twelve when he was passed over, almost forgotten, and not given an assignment. At the end of chapter seven, he feels entirely unsure of himself. He feels embarrassed at not getting any assignment, let alone a good one. However, when the chief elder finally recognizes that she skipped Jonas, she asks for forgiveness—which the crowd gives her as a ritual.
Then she says that Jonas has not been assigned but rather selected because he possesses an exceptional quality that only exists in the giver and receiver of memories. Jonas can “see beyond”—something that the chief elder says she cannot describe or explain. That causes Jonas even more worry:

Now was the moment when he would have to confess, to say, "No, I don't. I can't," and throw himself on their mercy, ask their forgiveness, to explain that he had been wrongly chosen, that he was not the right one at all (chapter 8).

Jonas doesn’t think he can see beyond, and he panics because he thinks that everyone will know that he was chosen incorrectly. He feels ashamed at being unable to do what the elders have selected for him, and he doesn’t understand what she means by seeing beyond. Then something happens to him, and he is suddenly sure that he can see beyond:

But when he looked out across the crowd, the sea of faces, the thing happened again. The thing that had happened with the apple.
They changed.
He blinked, and it was gone. His shoulders straightened slightly. Briefly, he felt a tiny sliver of sureness for the first time (chapter 8).

Jonas recognizes that these brief flashes where things change, where he sees color, are the gift of seeing beyond that the chief elder was talking about. He feels sureness in his assignment because he realizes that he is different than everyone else and that he does possess the gift that the giver of memories said he possessed.
His confidence is crucial because it allows him to put himself fully into the job of being the receiver of memories, but also understanding his unique difference from everyone else is what eventually pushes him away from the community. Jonas would never have stepped out with Gabe or tried to escape if he hadn’t felt confident in his own beliefs and his ability to see things the way they truly are.