If I haven't found the right extracurriculars, can I still appear to be a dedicated student?
A "dedicated student" is one who is committed to learning. Your particular areas of interest may vary from other dedicated students, but it is your commitment to those specific areas that are of interest to admissions officers. So, for example, if you really enjoy History and take all of the courses you can (and do well in them!), an admissions counselor will see a level of commitment that is important.
If you can supplement that commitment with extra curricular activities that complement your interest, that is wonderful! But it is not always possible. Some students, for example, may have to hold down a part time job after school, or look after younger siblings, not leaving much time in the day for other activities. Others attend schools in areas with limited opportunities for outside activities. So, if this is the situation for you, do not panic! It does not make you look any less invested in, or committed to, your education!
If you work or take part in an unrelated activity (such as a sport completely unrelated to your academic interest), create a resume of these types of activities. You may even use that "is there anything else we should know about you" section to describe how you take care of your siblings, or engage in hobbies after school. Your dedication and sense of responsibility to the things you do will be the thing that makes you look truly dedicated, not the number or types of things.