Downloading Half-Life is illegal, and considered warez, which isn't allowed on these forums.
Just buy it, it's very cheap compared to other computer games out there right now (you could probably get a copy for 10-15 USD).
If you also do manage to successfully pirate a copy of Half-Life, you'll need a CD Key, which is required to play online. You get one when you buy the game, and it is specifically there to stop piracy.
Just look around, you could probably even find a copy new or used on the Internet for half the price I mentioned.