that "brown stuff" is likely just old blood. many women will have this brownish discharge on occasion after their menses not just after their first period. talk to your mother or a trusted adult female -- someone who has been having periods for a long time or you can consult a medical provider. basically this can be perfectly normal. annoying? yes but get used to it. you are going to be having monthly periods for many many more years.
i thought my first period started 2 weeks ago but now theres brown stuff. at first when i first got it it was a lot of red blood at one time now its all this thick dark brown stuff is this normal? keep in mind this is my very first period