It's extremely important to research companies before an interview and, ideally, before you even apply in the first place. After all, you want to get a sense of whether it would be a good fit and what the long-term prospects of working there might be. (And, of course, you'll want to make a great impression in the interview.)
You may want to learn about their size (number of stores, geographic areas), merchandise assortments (hardlines, softlines, major brands, private labels), standards, culture, reputation as an employer, financial stability/outlook, and plans for the future. You may want to assess their current leadership and their capability for driving growth. You'll want to research job opportunities, what qualifications they look for, and how often they hire.
Let's get to it:
The first stop should be the company's own website. Read the "About" page for their mission statement, history, and values. Check out investor information and, of course, their career page. Sign up for their blog.
Check out their company page on LinkedIn and find jobs they've posted, recent updates, employee counts, affiliated companies, and first-degree connections of yours who work there. Follow them to see future posts.
Go to news.google.com and search for the company. Read all the recent articles, good and bad. Find out what they've been up to lately and what their prospects look like going forward.
Look up the company on Glassdoor and read employee reviews and get salary information. You may even learn about interview styles and sample questions from people who have been through the process.
Similar information may be found on Indeed, including open positions, employee reviews, and salary information. They even have a listing of the best places to work.
I know not everything you read on Wikipedia is true, but that doesn't mean it's all lies either. You can find some solid information there, particularly in the list of links at the bottom, many of which may be difficult to find elsewhere.
Visit their company page to see what they've posted lately. Read the "About" tab for an overview. Follow their posts to get a sense of their culture and community relations. How do they interact? How do they handle criticism?
Follow their Twitter accounts, including corporate and career-related accounts, to better understand their mission and to learn about job opportunities. Follow individual recruiters and check out who they're following as well.
Similar to Facebook and Twitter, following their Instagram page will give you some insight into their marketing, promotions, and community engagement efforts. Get a sense of their style and tone.
If you haven't visited one of their stores lately, make sure you get in there. In fact, visit as many as possible to get a sense of what they're doing now and what their standards are. You can even conduct your own secret shop.
Reading about a company and visiting stores are both important, but it can also be helpful to reach out to people who work there. Find them on LinkedIn and communicate with them directly to get their thoughts.
Researching a company beforehand can help you determine whether it's a good fit, whether they have room for growth, and whether they'll even be around in a few years. It can also help you get the job offer, since you'll make a much better impression on the interviewer if you're well-informed. Invest the time up front and it'll pay off later.
Best of luck!