More and more posts are popping up of new people trying out Dota 2 looking for assistance / want to learn the game. Rather then a ton of separate posts, why not have one topic.
To start off, Dota is an extremely high skill, competitive, and hard game. That being said, Dota is also one of the most strategic, exciting, and satisfying games to ever be created. It is also unique in that how much you enjoy the game will be based off how much you know / how skilled you are. If you dive right in, chances are you are going to have a bad time. So please please please, learn as much as you can, play some bot games, and ask a ton of questions before you begin to make a decision if you want to play the game or not. The first initial 100 hours will be rough, but everyone playing the game has been in the same point. But once you get over the initial hump, things will click and you will realize why people enjoy the game.
Rather then re-writing a guide, here is the best Dota guide ever written. Purge is one of the nicest and hardest working people in eSports, so please utilize his guide, his videos, and his stream to begin to learn the game. http://www.purgegamers.com/welcome-to-dota-you-suck#.UM4noG9E4ws
Some quick info that I would like to add on top of that. If you are new, here are the heros you want to start playing first. They are: Tidehunter, Venomancer, Sand King, Lion, Lich, Warlock, Vengeful Spirit, Crystal Maiden, Skeleton King, Centaur, Shadow Shaman, Windrunner, and Earthshaker. The reason being, all of these heros are generally easy to play but they each have a spell that is fantastic in helping your team win the game. In Dota, abilities will do the same set amount of damage as they say on the tool tip, they do not scale. So for example, a Tidehunter ultimate will be super effective all game long...no matter the items you get or kills you achieve. This is opposed to someone like Anti Mage, he is an agility based carry who is farm dependent. If Anti Mage starts off well and gets a bunch of items (Battlefury, Manta Style...etc) then he will absolutely crush his enemies. However if he does not achieve those items in a reasonable time, has a bad start, of doesn't get last hits...he will become an absolutely useless hero. As a new player you need time to learn the mechanics, the items, the heros, the map...etc thing that people are still learning a decade after Dota began. So start with the heros that allow you to be effect all game long without having the mechanics / knowledge required. You will have way more fun and will enjoy learning the game, rather then getting frustrated.
One last note, Dota is notoriously known as a game with a very hostile community. This is true, people can be very harsh on new players, but not everyone is. So do not let a few rotten apples ruin your time in the game. Please ask questions, utilize the guides out there, watch a ton of game...by doing all these you will have a way better time within Dota. It truly is one of the most beautifully crafted games out there.
If you are looking for more Dota info, check out the podcast I do called Dota Insight. We are working on creating guides for heros and catering questions / discussions towards newer players coming into the scene.