This is where you have to start looking at things that will teach you, or learn to break down what you want to do, search on those things and put your script together.
How to have 100 hp ? create a variable.
var myHP : float = 100.0;
if its less than or equal to zero ? you just wrote the script there. Use a conditional check.
if ( myHP <= 0 ) { //Do Stuff; }
you get sent to a menu ? // Do Stuff : Unity - Scripting API: Application.LoadLevel
This is not going to be a continuation of everyone teaching you to code for each step of your project where you need a script. There is a literal ton of learning material out there. You need to get the basics of programming down, and get familiar with some of the features of Unity. Make a lot of small projects, that only do one thing, then when you have some experience, start on your first big project. This is the much quicker way to get your big project done, trust me!
Here is a list of tutorials to get you going :
Start at the bottom and work up : Unity 3D Student - 3d Modelling
this is the YouTube link for the above as one playlist : Unity Tutorials - Essentials 00 - Projects - Unity3DStudent.com - YouTube
the Unity Wiki : http://wiki.unity3d.com/index.php/Tutorials
A list of resources : How can I start learning Unity fast? ( List Of Tutorials ) - Unity Answers Helpful page with information on using Built-In Arrays and Lists :
unifycommunity.com?
The unity wiki link above is very handy with lots of scripts and shaders too (just check out all the links down the left, and the tabs along the top : http://wiki.unity3d.com/index.php/Scripts )
Happy Coding =]
http://video.unity3d.com/video/7720450/tutorials-using-unity-answers
the FAQ appears at the top of the page : Frequently Asked Questions - Unity Answers
also : How do I ask a good Question? - Questions & Answers - Unity Discussions