First of all, take a look at this thread
http://www.fm-base.co.uk/forum/football-manager-2015-tactics/205292-team-shape-roles-duties-guide.html
From it, you will learn basic team shape, and roles and duties required for each shape
Don't forget that if you use a flat formation, like 442 or 433, without DMC and AMC, you will need a more a fluid team shape in order to keep your players connected. A highly structured 442 would cut of your players from one another.

The above is an example of a LLM tactic. A highly structured team shape, to suit poor players at your disposal, and a more agressive team mentality, to play on physicallity, since you don't have players with good mental or technical attributes. TI's are also more agressive, we try to pin them into their own half, and keep play away from our goal.

The above is an example of defensive tactic. It is the opposite of LLM. You need roles that roam, strikers that defend, and defenders that attack. Since team shape is very fluid, you can opt for a flat formation. Ti's are naturally less agressive, as this is basically a more cautios, play on counter tactic.

The above is an example of possession based tactic. It's a mixture of LLM and Defensive tactics. You don't want players running ahead of ball to keep possession better, and you don't want defenders making direct passess, so the best mentality is standard. You need not just keep possession like defending tactic does, you need to regain it like LLM tactic does, so TI's are more mixed.
http://www.fm-base.co.uk/forum/football-manager-2015-tactics/205292-team-shape-roles-duties-guide.html
From it, you will learn basic team shape, and roles and duties required for each shape
Don't forget that if you use a flat formation, like 442 or 433, without DMC and AMC, you will need a more a fluid team shape in order to keep your players connected. A highly structured 442 would cut of your players from one another.

The above is an example of a LLM tactic. A highly structured team shape, to suit poor players at your disposal, and a more agressive team mentality, to play on physicallity, since you don't have players with good mental or technical attributes. TI's are also more agressive, we try to pin them into their own half, and keep play away from our goal.

The above is an example of defensive tactic. It is the opposite of LLM. You need roles that roam, strikers that defend, and defenders that attack. Since team shape is very fluid, you can opt for a flat formation. Ti's are naturally less agressive, as this is basically a more cautios, play on counter tactic.

The above is an example of possession based tactic. It's a mixture of LLM and Defensive tactics. You don't want players running ahead of ball to keep possession better, and you don't want defenders making direct passess, so the best mentality is standard. You need not just keep possession like defending tactic does, you need to regain it like LLM tactic does, so TI's are more mixed.