Well, there's quite a bit there that you'll have to learn.
Before we jump in, make sure you download and use craftbukkit or something like it that supports plugins! (http://dl.bukkit.org/downloads/craftbukkit)
Firstly, the best plugins are located at http://plugins.bukkit.org/.
Probably the most useful plugin I've come across is called Essentials (http://dev.bukkit.org/bukkit-plugins/essentials/). It has a TON of commands that you can use and flexible configuration options.
To answer the specific questions you asked:
1.) "How to make the radius of the spawn area wider?"
I don't completely know what you mean when you say this - do you mean make the area protection larger? If so, there are a few ways to do this. The best way is by using a plugin called WorldGuard (http://dev.bukkit.org/bukkit-plugins/worldguard/). It allows you to select an area and protect it, as well as many other things. To use WorldGuard, you also need it's dependency WorldEdit (http://dev.bukkit.org/bukkit-plugins/worldedit). WorldEdit is VERY useful in making large things. For example, if you wanted to make a bit flat area, you'd normally have to flatten it by hand. WorldEdit allows you to do that in about 10 seconds by typing //wand, selecting two points, and typing a simple command.
2.) "How to make a 'You are now leaving the spawn' message?"
If you installed WorldGuard, there's a flag that you can add that does just that. If you type //wand and select the two points (forming a giant, imaginary cube) around your spawn, then type "/rg claim <some name here>", followed by "/rg flag <the name you used in the previous command here> farewell <message when you leave here>", then that solves it (if that's a bit confusing you can contact me - details below)
3.) "How to enable the usage of lift signs/plates/buttons?"
This one is a bit tricky, because it involves some harder things. The only plugin that I know of that does this is called Falsebook (http://www.minecraftforum.net/topic/195785-pluginbukkit-falsebook-over-50ics-15/). It is a bit older, but hopefully not outdated. You'll have to experiment with that or read how it works.
If you need more assistance with Minecraft Server help, I'd be glad to! You can email me at spgearclub@dispansion.hopto.org.
