Help me, I'm being accused of something i haven't done! Please read details! What should i do?

Ok so my mother called me out this morning and showed me a burn on the windowsill of the conservatory outside and she wanted to know what it is, she later asked me if my friend had a cigarette there which isn't true because he was with me all day yesterday (why she asked?) and she thinks i'm lying and so does my sister, she later found some matches behind the bin, and i swear on my life i have no idea how they got there! She thinks it's me and she thinks i'm lying, so does my sister! I don't what to do because they'll never believe me, they never do! I'M ALWAYS GETTING BLAMED FOR EVERYTHING!!!! And I know my dad is going to go ballistic and he's gonna ground me! No one believes me. I really need help! Btw you can tell that the burn is never gonna come out. I tried cleaning it to avoid getting into trouble but it won't! please help i don't know what to do!