Automation of user's actions on web pages(sites)

I have a question about working with web on user's side. I need to automatize some user's actions on web pages(sites), but i don't know how to do it. I would have learned this myself, but i don't know what to start with, how to find information that i need. I mean i have no idea what programming language to choose to do this, or how this kind of applications or programs is called. Do i need to learn JavaScript or idk python or c++, or something else? Please, tell me anything you know about this, any information will be useful.