Am I a good singer.............?

Well judging from that video, I'll go ahead and say right now you aren't too great of a singer. You aren't the worst I've ever heard though, but there is room for a lot of improvement; you can do it. You're still 12 anyway. The truth is though, pretty much anyone can learn how to utilize their voice to sing at the fullest potential. All you need is practice and a good vocal coach. If you really want to get better the best way is to find yourself a teacher.