By "who started the war" are you just referring to who declared war? The war of 1812 was started when the United States Congress voted to declare war against the United Kingdom and the declaration was signed into law by President James Madison on June 18th, 1812.

Here's the declaration:

https://en.m.wikisource.org/wiki/US_Declaration_of_War_against_the_United_Kingdom

Unless you're looking for something more specific than that in terms of the reasons why this happened? I'd be happy to help with that as well.