Some webmasters or SEO companies start doing SEO instantly, they forget to understand about website's industry/niche/theme type and that will effect after some time.
understanding website's category and about website that is most important thing before start a SEO campaign. That is makes your knowledge updated so that you can get information about your keywords and your competitors. in short it is every important before starting keywords research.
What exactly you should do :
1) First you should read about website.
2) Now you know what about your website is. so go throw google and read about more
3) Now check related websites so that you can aware about your competitors too
Now you can go for keywords research
Best of luck