Problems to Start writing CSS Selectors

Hellow there.

Ima working in te proyect Fotomatic, but im realy get stuck.

This is my aprouch in HTML

<!DOCTYPE html>
<html lang="en">

  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="preconnect" href="">
  <link rel="preconnect" href="" crossorigin>
  <link href=";400;700&display=swap" rel="stylesheet">

  <link rel="stylesheet" href="./resources/css/index.css">

  <header class="encavezado">
        <li>Produc detail</li>
        <li>About us</li>
        <li>Follow us</li>
    <h1>FORMAT CAMERA</h1>
    <span>Emial us to request a demo and ve in our waiting list from <strong>Febuary 2024</strong> release!</span>
    <button>Join the wating list</button>
      <h1>Advanced, automatic, instant</h1>
      <h6>Shutter speed, apperture and flash output adjust automaticaslly</h6>
      <h1>Beautifully crafted inside-out</h1>
      <h6>From the paint outsid to the tiny screw inside, Fotomatic is crafted with love and 20-year of expertise</h6>
    <h1>Over 20+ filters to choose from</h1>
    <h6>Feed your creativity with 20 different filter designed by our eclectic in-house photographers!</h6>
      <div>Pic 1</div>
      <div>Pic 2</div>
      <div>Pic 3</div>
      <div>Pic 4</div>
      <h1>“It’s truly something that could create a brand new photography Renaissance”</h1>
      <h6>© 2016 Fotomatic, All Rights Reserved</h6>
      <h6>Designed in NYC</h6>



But i´m not figure out how to start writing th CSS cladess and selectors to styless the HTML.

Somebody can help me, or give me a clue, meabe som viedo ??