when should i start applying for colleges if my final exam is in march?

You can start whenever you want! The Common Application usually opens up in August, but you can definitely get a head start on your essays before that. You can also start putting together your resume of activities and asking teachers for recommendation letters in the meantime. Just give yourself enough time between March and October/November (when most early applications are due). If you want more time, you can apply Regular Decision which is in January for a lot of schools. Starting during the early summer months probably makes sense so that you have enough time and aren't stressed :)

Make a list of all of the colleges that you want to apply to that offer early admission or early action. Apply to all the school that offer early admission that you can, and choose your favorite school to apply early action for, since this is usually binding. For the rest of the schools, just make sure you apply by the regular decision, which is usually in November or December. For now, you can work on your common app essay, make your college list, and/or complete any extra classes or community service you want to put on your applications.