Welcome to Half-Life Answers. What would you like to know?

Go to the steam store and in the search bar type in the name of the mod you're looking for. If you don't know it, leave the bar blank and click search. You should be taken to a results page. At the top of the page is the search bar, next to it there is a category menu. Select "mods" and press search, and it should list every mod. Yeah, I know, they should really have those category options on the first page, maybe they will in an update.

There are only a handful of mods available on steam, and the non-commercial ones mostly just link to the website. ModDB is a great place to look for mods.