what should i tell my website devloper while making my website

i want to make a perfect website with on page seo.what should i tell should be done in my website while making my website

Tell him to do best website
Or even more better pay him more money!

This is not a question that can be answered easily. A "Perfect" on-site includes hundreds of ranking factors. You should probably ask him if he understands SEO and if he has made optimized websites like the one you want. Afterward, hire a professional SEO to have it checked if it's indeed optimized properly. That's the only way I can think of to be 100% sure that the developer will do a good job.

If it's being created from scratch, I would mention that you want to focus on optimisation and load times.

Ask him/her to make fully responsive.