To play DotA online in Garena Plus, you need to have a PC, Warcraft III (with latest patch i.e. TFT Version 1.24e and latest map which can be downloaded from GetDota.com) installed on your computer and a good internet connection to play smooth games. If your internet connection is not good enough, you may experience delay or lag in the game.
Next thing you need to do is install Garena Plus Messenger in your computer. You can download a fresh copy of Garena Plus from Garena.com.
After the messenger is downloaded, install it and register an account here.
Assuming that all the above mentioned tasks are completed, just follow the following detailed step by step instructions and you shall be able play DotA online with your friends or other players.