Just to add to what Alexis said:
The HL SDK does not yet contain any of the 25th Anniversary game code changes, such as implementation of func_vehicle. So if you want any of these features in your mod you will just have to wait.
As for what
is there:
After downloading the HL SDK from your Steam Library and pressing Launch it will open up Hammer, the level editor. This is only a small part of the tools and content the SDK provides which you can easily access by right-clicking the shortcut in your Steam Library, and go to Manage -> Browse local files.
In the main directory you will find various documents about HL modding, and several sub-directories.
Most of the tools sub-directories contain very outdated tools which you can find more updated replacements for in the
Tools and Resources page.
Models sub-directories will have source files for many different models, while Prop Models is a library of lots of decorative models you can use for your maps.
The Texture Wad Files directory has the standard set of WAD files that comes shipped with Half-Life.
If you're interested in the game code, you can look at the Github repository Alexis linked or you could head on over to the
Half-Life Programming - Getting Started guide.