Well I think you should never the less google a bit around for some linux introductions, especcially for things like quesiont 2 etc. because they are improtant basics...
1.) try wget. just enter "wget URL" (without ") and see what it does... you can even finish a broken download by using wget -c for more see: man wget or wget --help
2.) normally if you install the webserver (or when it is already installed) it will be automatically started. The scripts for starting daemons are in /etc/init.d
Which daemons an script shall run depending on your runlevel (for a server normally 3) can be found in /etc/rc.[runlevel]/ here are symlinks to the script from init.d. The S stands for start and K for kill the number afterwards for the priority when to to what. There is also a tool for creating such stuff but I normally do that by hand, maybe sb. else can help here.
3.) The adress must be given by your provider but I guess if you set it up you should already have it? You can check your ip with ifconfig interface (normally ifconfig eth0)
4.) First of all you should split things up here. Nameservers on your machine are not necessary for hosting sth. E.g. I use my DNS-Providers NS (so I don't need to care about anything here).
For hosting multiple sites you just create the webs with ISPConfig. You can point as much DNS A Records / CNames as you like to your machine...