I have an idea for a large company. Should I patent it first before I contact them?
You cannot patent a mere idea.
However, if your idea is at the prototype stage, a patent might be useful to you at least from the perspective of having some chance of preventing the large company from simply taking your idea as their own when you disclose it to them. An NDA (non-disclosure agreement) is also useful but not as useful as a patent to help to prevent this.