Valve announces that the next big sale begins on Steam in just under a week. On March 16 at 10AM (PT), the Steam Spring Sale will begin. This sale runs until March 23 at 10 AM (PT) and will feature plenty of savings.
I mean, it's a sale. You know what these are by now and if you don't know what a sale is then you are probably too young to be buying anything in the first place, especially buying things online.
Valve put together a video for this sale. You can watch it, or not. The sale itself will be featured on the main Steam page when it kicks off. It'll be really hard to miss.
If you're looking for something a little different, maybe check out Fanatical. They're running their own Spring Sale right now. They're doing this thing where you can make your own bundle deals. There's flash deals each day. More often than not, they do provide Steam keys for games. Not all the time, but like a lot of the time they do.