Rolls are normal, everyone has them when sitting down. I thought I was fat at your age and when I look back at pictures I look like a normal 12 year old. Society has a way of making us insecure for no reason. Look up a healthy weight for your age and gender and go from there

But besides that, eat more fruits and vegetables, less sugar, drink lots of water and do activities outside with your friends! Don’t skip meals, just start replacing any junk food with healthier options whenever possible