Get the uniform. The basic uniform is a white button up shirt with black pants or a black skirt. You will need to get an orange and red striped tie to accompany the ensemble. Shoes are basic black, and the girls are seen wearing knee high socks with skirts.
Fashion a cape. Capes are necessary to dress like a Hogwarts student. Get a black billowy cape and fashion a Hogwarts crest onto the left lapel.
Wear a dark blue vest. Girls and boys alike will walk around sans cape, and when they do they wear a sweater vest. While wearing the vest they keep their ties on.
Make it your own. Every student, even young witches and wizards, make minor adjustments to their wardrobes. Some leave their white shirts untucked or change the way they tie the ties they are forced to adorn. They also have a winter look that includes a wool scarf with their house colors.
Carry a wand. You may not be able to go to Ollivander's for a new wand which chooses you, but you can buy wands on the Internet or a wizardry hobby shop. Carry your wand in your cape or pants pocket when you dress like a Hogwarts student.