Ok, I'm rereading CoS right now and in the beginning of the story when it's talking about what house he was in last year it says exactly "...-but he ended up in Gryffindor, along with Ron, Hermione, and the rest of the Wealseys."
Sorting hat thought harry would be better in syltherin but Harry kept saying "not slytherin" so the hat let him choose thus Harry went to Gryffindor because it suited him as well.