You have to pass the Judges, if you don't you're stuck at 999th,
as soon as you pass your Pro. Debut you are immediately boosted up to #5 ranking, but you have to work a bit to pass the judges, so my advice? Practice practice practice, pracitce as much as you can with your band, it doesn't matter as much to the judges if you have mad skillz( I have passed with skill points around 300) as long as you and your band co-operate.
one last thing? after passing the Pro Debut, which really can cause a bit of trouble because of the difficulty, the rest is smooth sailing. it is amazingly easy to get to #1 after the Critical Judges.
hope you are having fun with your Music Star! it's one of my favorites.