First I like to say welcome to the hive
You will find that it have many many nice features
Enjoy your stay 
Now for your questions (Which you actualy shouldn't ask here, but well I snwear them anyway)
1: There are several programs, I give you some names then find the one which matches you best (search google for them or look in our tool section)
Milkeshape (Model program, costs money but cheap)
Blender (I believe you can find it for free)
Gmax (Also is possible to get for free, there was a link somewhere on hive)
3DStudioMax (Best program, but also the most expensive (advice search google))
2: Actualy answeared it above, and no the only reguirements is your own skill, pratice makes perfect you know
But some additional programs will be needed to really make models.
3: Just click on the text which says
Models in green (look in the left cornor of screen)
4: No its free, although you always can consider donating to hive, but that is only an option for those who really love this site and want it to stay here forever.
Finally I am going to list up some programs you might need to really allow you to model:
Magos Model Editor (this allows you to add particles (explosions, dust so on), edit textures and see your model similar.)
WC3Viewer (This allows you to see your model, and most important it allow you to make your own skins/texture in Photoshop and convert them from tga to blp fileformat)
Photoshop (this is needed if you want some good looking skins/textures, but you can always use the ingame skins/textures -> use magos model editor to find them.. read a tutorial and you understand (Search for the program with google))
And you might also need oinkwinklers tools. (They are usefull to, read about them by searching google and you will understand)
Anyway most of those programs can be found in here on the site in the tool section (ALmost the same place as where you find the models, just click the green text saying
Tools).
Hope thats all. Anymore questions just ask them in the proper forum for it.
