FIFA 13, like earlier versions of FIFA, involves a lot of strategic planning if you want to win consistently. A lot of people just play with the default formations and leave them how they are. While there's nothing necessarily wrong with this, it is a far better idea to focus on which formations will best complement your playing style in FIFA 13 and decide accordingly.
I'd also like to take this opportunity to tell you that you should really consider the Premier FIFA 13 Guide. In my personal, unbiased review, I talked about how the guide provides an overload of information on certain topics. If you're interested in using formations creatively and to your advantage, that alone makes it worth buying the entire guide which comes with loads more. Each possible formation is explained comprehensively and the author evaluates the strenghts and weaknesses of each one.
The most commonly used formation both in FIFA 13 and in real football, the 4-4-2 is a balanced, conservative formation. If you don't want to get too non-conventional with your formation, you should just stick with this. This is not to say that the 4-4-2 is a bad formation; the 4-4-2 provides a well-spaced out playing field that will lead to a decent number of open passing lanes to utilize, but you won't see a lot of clear positional advantages over your opponent while he will see few over you.
While the 4-3-3 is very conservative like the 4-4-2, the 4-3-3 is a bit less balanced throughout as it focuses on strong wing play. If you like to dribble with your wingers for whatever reason, and/or you are playing with a team that features quick wing players, then you should consider using the 4-3-3. If things align in such a way that leads you to use the 4-3-3, you should see a lot of goals coming from quick sprints down the sidelines and crosses into a forward to head towards the goal.
The 4-4-1-1 features a balanced midfield and defense, but presents an entirely different attacking strategy. With a 4-4-1-1, you have a deep striker who makes a lot of runs and is the primary target on crosses. Typically, you want a big, strong guy here who can shove off defenders or a quick striker who can sprint away from them. Next, the second-striker has a mostly free role to move all around the attacking third and distribute to the main forward. This formation is especially useful in FIFA 13 because defenders are particularly prone to penetrating runs and through balls.
The 3-5-2 is a unique formation, at least in comparison to the others mentioned above. The 3-5-2 puts only 3 full defensive players on the field, while the rest of your arsenal is focused in the midfield and forward positions. While having only 3 defenders may seem like a huge disadvantage, it really isn't if your playing style works well with it. What kind of playing style works best with a 3-5-2 in FIFA 13? If you like to possess the ball throughout midfield and wait for the perfect opportunity to actually attack the goal, then you may be surprised at the amount of success you'll have with the 3-5-2. Teams like Barcelona actually work great with this in FIFA 13 as you can fit a ton of great passers in midfield (other formations would force you to have to bench a few) and really work the ball well.
This formation is the much riskier cousin of the 3-5-2. The 3-4-3 is an extremely attack-focused formation, with a 4 player midfield while still maintaining 3 attackers. While you'll feel a significant advantage over your opponent when attacking, you will also certainly feel the problems with a 3 man defense like the 3-5-2, except this time you have even less midfielders to come bail you out. If you're good with possession and defense, then give this formation a try at least.
The 5-3-2 should only really be used when you feel you'll be under a lot of pressure defensively, for whatever reason. If you're probably going to be playing a balanced, back-and-forth game, then there's really no reason to use the 5-3-2. Because FIFA 13 will put you up against players who are significantly better than you in game modes like Head to Head Seasons and whatnot, you should familiarize yourself with the defense-centric 5-3-2 formation. While this formation is predominately defensive, don't be surprised if you still come up with a counter attack from time to time as they'll be focusing a lot of pressure on your back line, meaning less players will be back to defend a quick counter.