The introduction section is the shortest. Start out with a Cover Letter and a Title Page. Keep the Cover Letter brief: simply write a personal introduction to explain who you are and provide your contact information. The Title Page should be exactly what it sounds like: a title that introduces your proposal and provides a clear message about the ideas and/or services you are pitching. Some examples might be "Proposal to Create a New Executive Assistant Position", "Proposed Temporary Services to Benefit the Stuart Corporation", "Executive Search Services Proposed for Jameson Company", or "Suggested Candidates for the Vice President Position".
In proposal writing, your first step should always be to gather information about the party who will judge your proposal. That's because you want to present a proposal tailored to that party's specific needs and knowledge level. In other words, you need to put yourself in the other party's shoes and look at the situation from that party's point of view. If you are pitching to your boss or your company executives, you might already understand their positions and their concerns. But if you are pitching to people at another company, then you will need to do a bit of work researching who they are, what they do, and what their needs are. Yes, that research can take a bit of effort, but putting in that effort makes your proposal much more likely to succeed.