Those hacks are somewhat harder to make as minecraft constantly updates, and require knowledge of Java.
Currently most hacks are permanent minecraft hacked clients. Start off by decompiling minecraft.jar and then look around in it. I'm sure there are tutorials about it in the internet. I would write one but I'm not familiar with Java at all, maybe someone else does :-\
But if you find it too hard, try ereaser hack for minecraft 1.2.5, it's very powerful and contains all you need. Available from mpgh
I hope you try to decompile it though, and learn something, good luck with it
(It's not always best, but it's good to learn and discover things on your own)