hi
that is easily doable... I would use an LCD unless you like to wire a lot. Serial LCD's are asier to wire than parallel LCD's. Other than that you just need to connect a switch (see button tutorial) and a relay or high-current transistor for the Car horn (Google arduino + relay).
You will also need a power supply or battery capable of putting out the current for the horn: something like 3@ at 12V.
When you start to build this, just do it one piece at a a time:
- get LCD working, see if you can print messages to it;
- get button working- can you print messages to LCD in response to button?
- get horn working: can you control the horn on-off with code?
Once the answers to all these questions are yes, you can integrate the final code to make it all do what you want.
D