I forgot to say, Welcome. As marmak3261 has stated, there are various ways to make stencils, but since you specifically want to use mylar, you should pay special attention to the “Mylar stencil cutout” post in Beyond the Manual that he links to above.
As for your question regarding Apple computers, the Glowforge doesn’t care what computer you use. You must design your stencil in a vector graphics program such as Illustrator or Inkscape using your computer or ipad. The settings for cutting or engraving are entered into the Glowforge interface not in your computer design software.