For legal streaming you have sites like Crunchyroll, Neflix, Funimation & Amazon, in YouTube you can also find some of the Funimation series in its channel (though I personally can't watch them, I guess its locked for my region). Crunchyroll have some if not most of its series free to watch for non user and free user, though region lock might apply, and series that are in emission are also free to watch except for the most recent episode in which case you need to wait a week for it to be free, unless you are premium of course. Crunchyroll also offer manga, aka the Japanese version of comic books, but that is mostly for premium users. And I don't know about the others, so I can't talk in more detail of them.
If you don't care for the legality of it, and just want to watch some anime or the serie you want is not in the previously mentions sites, then sites like KissAnime (the .ru is the good one, to some definition of good, don't confuse it with the the imitator, yes it has imitators) are the place to go to watch almost any anime ever made, but in that case is recommendable the use of adblocker like uBlock Origin to avoid potential virus load ads.
You can also find series in YouTube, but if it not from a legal source, then chances are that is heavily alter to avoid the copyright system
Thank you for adding to Thatanimesnob's recommendations. I shall look into the ones you both recommended but will hold over on the others as don't want to get it wrong and end up being hacked!