you need WorldCraft 3.3, i understand there is a new version out called Halflife hammer editor or something like that. Go to
www.planethalflife.com and they have, on the left hand side a menu and towards the bottom it says modifications, you want to click on the Editing link. That will take you to a new page and you can select what you want from there. For modeling you will want Milkshape, just do a search for it at
www.google.com and you should fine where you can dl the unregisterd version for free. Hope you have a good time mapping. Oh a note with Worldcraft, you have to find a compiler also, Zoners tools is i guess the best one but i cant ever get any compilers to work. Good luck.