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In League of Legends, a free to play online game, teamwork is the key to success. When you dont work together you fall apart, one teammate with be killed consistently, your enemies will become more powerful, and your entire team will fall because of it.
To be a strong team, you need to have a well balanced group of characters, and you all need to know how to play your type of character correctly. When starting in a 5v5 game, you need to fill a few basic positions
Tank: A higher health character who can take a beating. Buys items in the game that increase armor and magic resistances, making them tough to take down. Basically, these characters should take all the damage for your weaker players, and should work more on assisting those teammates instead of getting kills on your own. Characters like Rammus and Shen are perfect for this.
Dps (ranged or melee): Dps stands for damage per second, and the job of these characters is to destroy the other players, their turrets, and basically anything else in their way. They do good consistant damage, but their drawback is that they usually have very low health, especially ranged. These characters should buy items for attack speed, attack damage, and critical strike. Other characters do mass damage in a second, but afterwords cant do much until their cooldowns reset. These characters are called "nukes", and are played more to hurt enemies than towers. These characters should focus on buying items that increase their ability power. Let your tank take the front line, and while your enemies are trying to hit him, take them all down. Tristana and Yi are good dps characters, while ones like ryze and annie are nukes.
Support: As shown by the name, a support character is given to helping the other characters through things such as healing, mana regen, and stat buffs. They dont generally do too much damage, but can be amazingly helpful when laning with them. As long as you arent killed immediatly, the support should be able to heal you and send you back into the fray without you having to recall back to base. Also, they sometimes have stun and slow moves that allow dps characters to do their job. Sona and Taric are considered good support characters.
The team should consist of something like 1 tank, 1 support, and a mixture of 3 dps, intermingling the attack damage and ability power. This gives you a goob basic assortment of characters to fit every position you need filled.
Now that you know these characters, you need to know how to form them into a team. It should be like this for each lane.
Middle lane: 1 character, ranged dps. This character is referred to as the "carry". They level up and gain money faster than characters in other lanes, as they do not have to share minion kills or experience with others. This player must be very cautious, doing what they can to gain the upper hand over the opposing carry. Sometimes, players from other lanes will sneak over to double team you, thus giving them a distinct advantage. Your teammates should be ever on the lookout, and when a player in theri lane goes missing, they should call "mia", or missing in action, to let you know that you need to be aware of danger.
Top and bottom lane: Tank and dps. Support and dps. Have each dps go with either a support or tank. This way, the dps has either a tank to hide behind or a support to heal them when hurt. This way, you should be able to keep yourselves strong and balanced so that your dps may spend more time attacking and less time hidin while your health regenerates. If you have good control of your lane, have the dps go for a sneak attack on the opponents carry. This is called a "gank", and like i said earlier, things can go downhill fast when in a 2v1. If their carry is not paying attention, you should be able to help your carry take them down, giving your team more of an advantage.
Please note, this is a very basic guide, there are many other strategies and chracter combos to try, but if youre a newer player try and stick to this. If you have trouble playing support characters, as it can be at first, just sub in another tank. Good luck