Help newbie

[Ken Holmes]

Can someone help me please
I AM NEW AT THIS
I read the DNS howto online and they always confuse me with all these sub domains like jerry.springer.foo.com 
oprah.winfrey.foo.com  bla bla bla
And the DNS and Bind Book do the same
I wish someone could explain to me if you have 1 PC that is connected to a net connection and that PC is DNS for virtual domains how do you set up DNS
I have the DNS and Bind book It kinda confuses me a little, but I will read
it and understand it.
I need imediate help.
I have a primary dns which we will call mydomain1.com that DNS is handled by
the ISP.  Now I have setup Linux Red Hat 6.0 and sendmail and DNS and BIND
OK so I am only allowed to have one IP they are cheap.
So now I have multiple sub domains. I registered with Internic.
At first everything went fine. I was lost about what to do, but I got them
working so now I have www.mydomain2.com as a virtual domain.  So now I
registered like 2 more and number 3 works www.mydomain3.com and number 4
worked right untill tonight www.mydomain4.net .  Now I can only access it by
typing mydomain4.net which is no good because the people want to type
www.mydomain4.com
Here is what this domains zone file looks like

@               IN         SOA    virtual.mydomain1.com.
hostmaster.mydomain1.com
                        1999080903 ; serial
                        3600 ; refresh
                        900 ; retry
                        1209600 ; expire
                        43200 ; default_ttl
                        )

mydomain4.net. IN      MX      10      mail.mydomain1.com.
@               IN      MX      5       mydomain1.com.
@               IN      NS      mydomain1.com.
@               IN      A       207.179.141.20
www             IN      A       207.179.141.20
www             IN      NS      mydom,ain1.com.
mydomain4.net  IN      MX      5       mail.mydomain1.com.


Here is a file from the one that works I know this one is screwed up, but I
don't get why it works and the other doesn't above
Even if i try and copy this one to the file name above and change the values it still doesn't work
@               IN      SOA     virtual.mydomain1.com.
hostmaster.mydomain1.com
                        1999080903 ; serial
                        3600 ; refresh
                        900 ; retry
                        1209600 ; expire
                        43200 ; default_ttl
                        )
mydomain2.com.               IN      MX      6
mydomain2.com.               IN      MX      10
mail.mydomain2.com.
@               IN      MX      5       mydomain1.com.
@               IN      NS      vmydomain1.com.
@               IN      A       207.179.141.20
www             IN      A       207.179.141.20
www             IN      NS      mydomain1.com.
www             IN      MX      5       mail.mydomain2.com.

Please help me with this problem whether it be a total overhaul of my DNS I
really need to get this working

Thanks for the help and sorry about the length of this message, but I am getting real desperate at this point