How can you get free games from Steam

Steam has a variety of free-to-play games. They are all listed in the store front with a price of $0

You must have an active steam account; the steam software installed and than simply browse the store for free games

The steam community forums also keeps a list of free games (see related links)

If you are asking: how to get games for free that would normally cost money: unless you have friends that you can ask (who would than give you credits on steam), you can't get those for free, you (or someone you know) will need to pay for them (or buy a steam gift card at a real store if you are trying to avoid using your CC card online)