How does someone get into writing short stories and work his or her way up to novel-length books?
All that fancy, schmancy talk is just to say that they're different. It's not as though a novel is a grown-up short story, like a man is a grown-up boy. It's more like a man and a watermelon, or some other unrelated thing.
But, to specifically address the question, you can most definitely try your hand at shorter works first to exercise your writing muscles and find a sense of completion for a project. That sense of completion certainly can help a beginning writer build the confidence that "Yes, I can do this!"
Another option is to see your novel as a series of short tales, and take them one at a time.
Of course, if your goal is to build your confidence, nothing beats having a short story or two published in an anthology or magazine. That, more than just about anything else, will instill in you the confidence to keep going and have the guts to tackle a longer work.
Good luck and happy writing!