Start visiting college campuses during the student's sophomore and junior years. These visits will help you both get a clearer picture of the campus as well as what the school really has to offer, housing arrangements, and other details. It will also give your student a boost of encouragement as to the college life which is right around the corner.
Submitting college applications usually starts early in the student's senior year. Be advised that some colleges have very early due dates; as early as October. Find out what your could've prospective college application deadline are.
The student will probably also have to write and submit some from of an essay to go along with the application. Encourage your child to start writing drafts of the paper during her junior year so she has plenty of time for re-writes to perfect it.