You need to start by thinking about the benefit for the advertiser. WHY might they want to advertise on your site? What's in it for them?

Once you understand your target audience and how they make decisions about online advertising, and about how you deliver measurable results for them, then it's time to develop your proposal.

If you already know those things, perhaps you can share them with us, so we can respond more directly to your request.

I would look at similar sites and try to find out as much as you can about how they sell their advertisements. Look at the information they provide on the site users, pricing, etc., and then try to figure out how your site is similar and different and how that would impact what you would do.

Keep in mind, an easy way to ad advertisements to your site is to add something like Google Ads (called Google AdSense).

Michael and Peter are both right in their responses. Also keep in mind that advertisers buy numbers - exposure to large audiences. Unless and until your website can claim numbers that will impress an advertiser - numbers at least in the many thousands - no proposal in the world will get a significant advertiser to sign up with you.