PA28 student pilot reports inadvertent toe brake actuation during landing resulted in a runway excursion.

Narrative

After completing my second solo cross country flight; I was performing my fifth landing and skidded of the runway. The landings were all normal types of landings with normal airspeed indications for the downwind; base; and final legs. My approach and flare was good and normal; but upon touchdown; the left brake was used. This may have been because of how I put my foot on the rudder which was high. After touchdown; because my foot may have been on the brake; the airplane made a big left turn due to skidding. I was surprised and released both toe brakes; but the aircraft was heading off the runway. Then I used both toe brakes to stop the airplane; but it did not stop. Finally; the airplane went off the runway. There was no injury to myself; and no damage to the aircraft; or airport. Winds were light and variable; with clear skies. This was my seventh solo flight. To avoid doing this again; I will be more vigilant with my foot placement during the landing phases. My instructor emphasized heels on the floor when landing; and I will make note of my heels on the floor during the approach. I will also practice several landings with my instructor.