Even the most basic company website offers some form of information to help you prepare for a job interview.  Refer to sections like "About Us" to learn about the basics of the company, "News/Press Releases" to be informed about latest news and 

developments at the company, "Management" where you can become familiar with important names and see details of who heads the particular department you want a job in, and "Locations" so you see where offices are located.

If you're looking to really impress, also read up on industry news and know what's happening in the space so you can have an intelligent conversation.  Employers are impressed by applicants who not only know its business, but about its market and competitors as well.

LinkedIn is much more of a professional social media platform, so you can expect its page to include business-oriented news that can help you prepare for the job interview. You can even search for people who used to be in the company and find the individual who might have held the job you are seeking. 

Not every employer will have a Facebook, Twitter, or Instagram account, but for those who do, they're easy resources to go through to pull a lot of information quickly. Through what they post, and the tone of their postings, you can learn a lot about a company and its company culture.