Note: Before you read this thread don't expect that I will provide you with legit tutorial. Don't be a shit on asking for legit serial you must have a valid KEY(beta,retail,invite). Follow legal rules. Say NO to piracy.
Step 1:
Install Steam.
Open your browser and Go to Steam's website
(Welcome to Steam) and
download the setup file (1.5MB).
Once the installation is finished, it will update itself,
which takes around 5 minutes or more.
Note: New users make an account by registering.
Step 2:
Installing the game.
Library > Add a Game > Activate a Product on Steam.
(It will open a dialog box for you to follow.)
Enter your Dota 2 beta key.
If it's genuine, then there should be no problem for
Steam to accept it and for you to start the download.
Download Dota 2 via Steam by clicking Install/Update.
Depending on the speed of your internet connection,
it might take anywhere from half an hour to overnight.
An average of 500kbps will download the game in
around 1 hour and 15 minutes.
Step 5:
Update your game.
Regular updates are on every Thursday.
Click Play. If there is an update, it will automatically
receive it before you get to play the game.
Updates are around 200MB or less (for smaller updates),
so it won't take much time.
Enjoy DOTA 2