Yes for the best results when taking your blood pressure you should sit up right without slouching with your back against the chair and your legs uncrossed.
Your arm should be as straight as possible and you need to be very still and avoid moving around or talking while taking your blood pressure.
Moving around and talking can mess with your blood pressure reading so be quiet and still until the blood pressure reading has been taken.
It's also best to use your left arm while taking your blood pressure but you can use either arm if you choose to do so.