You would be better off buying Heroes of Newerth, since its better than DOTA in pretty much every way. FYI HoN is a game based on DOTA, and a lot of the heroes and items in it are direct ports from DOTA. Consider it an upgrade of DOTA. However, to buy HoN, you'll have to get a relative or friend outside of Pakistan to use a credit card to purchase it online for you. Once thats done, you can download the game and play, ie your relative wont have to parcel you anything, everything is done online.
If you cant buy HoN, and want to try out DOTA, just visit garenas website. They have all the information you need to get started. I'll give you an overview though: First, you need to have warcraft TFT installed, ie original game + expansion pack. Then you need to upgrade it to the patch people on garena are using, which is usually the latest one. After that, you need to download the DOTA map into your war3's map folder. You'll also need to install garena, specify the games path in its options, and use it to launch the game.
If you end up buying HoN, lemme know, we can play together