When you have created a Puppeteer page object:

const page = await browser.newPage()

You can use the screenshot() method on it to save to screenshot.jpg in this case:

await page.screenshot({
  path: 'screenshot.jpg'
})

Add the fullPage option to screenshot the whole page:

await page.screenshot({
  path: 'screenshot.jpg',
  fullPage: true,
})

Also see my full Puppeteer tutorial