Navigate to store.steampowered.com/about/. Here, click on the green ‘Install Steam Now’ button.
Once the download has finished, run the setup file and click ‘Run’ on the the window that pops up to start the Steam Setup wizard. Proceed to the next step once installed.
Assuming you don’t already have an account, click ‘Create New Account’. Once created, run Steam.
Navigate to the ‘Store’ tab.
Type ‘Dota 2′ into the search bar and click on it in the search results that show up.
Scroll down the page until you find the green ‘Play Game’ button and click on it.
Click 'Next' to begin downloading DOTA 2.
You will see ‘Dota 2′ coloured orange in your list of games along with its current download completion percentage and your current download speed.
When ‘Dota 2′ turns from orange to white, the game has been fully downloaded. Click on it and Steam will run some further installations and then you’re ready to go. Have fun!