Most graduate schools only admit students at the beginning of the fall semester. In the summer before and early fall of your senior year, write to universities that interest you, requesting information about each school's English graduate programs and application forms for admission and financial aid.
Information is free.
Although no grad school is going to make you decide whether you want to specialize in 16th century sonnets or Ogden Nash at the moment when you apply, some schools are better in particular areas than others. Ask the professors who teach the courses which interest you most. If your interest is general, ask us about schools which are good in most areas. Think about where you do or don't want to live for the next five to eight years. Look at the information you get and see which schools offer the best programs of study and the best financial aid.