Nba fantasy - managing roster?

I am playing in the Yahoo nba fantasy head to head league for the first time. I understand that it goes for a week from mon- to sun night. I also noticed that its the stats for 9 players. SO...I need help understanding the time table. Is it wise to leave your best 9 playing? or do you need to put guys in based on who is playing that day? How often does it tally up the points?