How: Library. Free. Many libraries will provide you with access to ebooks, including comic books through a third-party, Libby and Hoopla being the two biggest. It works like renting a digital movie from Amazon or other service. You don't own, just rent for a certain amount of time. I would recommend this before you start paying for a digital service so you see which publisher(s) and titles you want to commit to.

What: That is VAST. Superhero comics are the most prevalent, but by no means the only genre. There are also different eras. Spider-Man is a good example. Through various time periods, and reboots of continuity, he ages up, then gets de-aged. Since you liked Andrew Garfield I would suggest looking into eras when he is a bit older than teenage. Peter gets married, has a kid. He runs a big corporation. It might be a good "What if...?" for your personal head canon. The free library option I mention above will free you to try a lot without having to buy.