It is very important to look at matchups for your players, the team your players are playing against. This is one thing that can separate a good fantasy football owner or a bad one. Say if you have 3 really good wide receives, which is not uncommon, and you're not sure which ones to play. One of the most important factors to whether or not they score alot of points is the defense they are playing against. Look at numbers like rushing yards and passing yards allowed for the defense you are going against.
Obviously going against a team that allows alot of passing yards is a good thing. This is what you want to look at first. You want to also see the defense's difference between passing yards and rushing yards. Coaches are going to look at these numbers as well and if they see a big difference between the two the coach will try to exploit this. Be sure to bench the players that are going against strong defenses in their respective category.
Don't strictly go by these numbers though. I will almost always choose my 1st round drafted running back to start, even if he is going against a great defense. Especially since my backup running backs are not that great. There are many factors in determining who to start week to week, be sure to look at the defenses those players are going against.