Waseda is a big university, with many different schools that operate fairly independently. I have just started teaching at the School of Law this year, so I am just beginning to learn how the School of Law works. I really can't advise people who want to study theater or history, for example, at Waseda. However, I can give some general advice and links to information available on Waseda's web site.
Students apply to and enter a particular school or program within the university.
You can find out about the programs and classes available at Waseda on-line. The web page Programs for International Students has links to English home pages of the various schools and programs. You might also want to check out the English version of the Waseda bulletin. Waseda has a Center for International Education which helps foreign students who want to study at Waseda and Waseda students who want to study abroad. Most of the classes taught at Waseda are taught in Japanese, so to enter most of the schools and programs, you will need the ability to speak, read, and write in Japanese.