@Desmond @khalil1210 Thank you! I shall keep this in mind.
One more thing, epubs doesn't work in Kindle?
Once you set up Calibre, click on
Add Books and pick out any free ebook files you've downloaded. They'll appear in the Calibre library. Then click on an entry in your Calibre library and choose Convert Books from the top bar.
In the upper right-hand corner, make sure to set the Output Format to Mobi. Click OK, and wait until the Jobs number at the lower-right of the main screen goes from 1 down to 0. Now select the title again and click Save to Disk.
Pick the folder you want to save it in. It'll create a subfolder with the name of the author. In Explorer or Finder, drill into that folder until you find the Mobi file within, which you can email to your Kindle or drag and drop onto it using a USB cable.