I'm still confused abouth nameservers and are begging for help.

Where do I get my own dns1.mydomain.com and dns2.myname.com? Do I get this from my domain registrant (enom, namecheap, ...) or do I get this from my hosting firm (where I have a reseller acount)???

I would like to offer my customers (I design web sites) domain registration and web hosting.

Let's say that I registered my domain at one firm and have a resseler acount at another firm. Is it possible to use my own nameservers names???

Go to your host and tell them that you would like to have personal nameservers.

They should give you 2 IP's and the correct nameservers, there may or may not be a charge for this.

You will then have to go to your registrar and register the nameservers, once that propagates, you can then assign them to your domain or any others.

I get it now.
My host should have two namenameservers with 2 difrent IP and that I use with my subdomains dns1, dns2 (with my domain registrant).

I have one more question. How is it done that a domain (mydomain.com) name is pointing to a specific IP?

In WHM, you can choose which ip that domain will be on while creating the account, and also can change it by using the "Change Site's IP" link.

but what if I'm using resseler space. Do I have to modify the IP for my customers or not if I use my nameservers names?

If you do not have root access to the server, ask your host administrator to put the two assigned nameservers into your reseller account setting. Make sure they also make proper modifcation to your clients' websites.

After they have done so, you simply create like as usual and everything will work.