It depends on your league's settings. If you're playing in a league with weekly lineups, be sure to check how many games each player on your roster has on the schedule for that week. Generally, you'll want to start your best players most weeks, but if one of your stars has only two games, he could be out-produced by a lesser player whose team plays three or four games that week.
In leagues with daily lineups, you, of course, want to start players who have a game on that day. Deciding between those players can sometimes be difficult, so you'll want to consider factors like recent performance, opponent strength, and statistical projections.