There are two popular sites for watching movies online. The first is Netflix and the second is Hulu.
Netflix has a large library of movies, TV shows, and documentaries to choose from. It also has an original content library that only subscribers can access. This is where you will find Stranger Things, House of Cards, 13 Reasons Why, and more.
The downside to Netflix is that it doesn't have any new releases or the latest movie releases available for streaming so you might have to wait a little while before you can watch it on Netflix. Hulu offers a lot of new releases but they don't have as many choices as Netflix does in terms of TV shows or documentaries so if you're looking for something specific then it's likely not going to be available on Hulu either.