I've already directed you to the links with all this information, but here's a bit more detail:
First, have you set up DNS servers under DirectAdmin? If not, look
here and
here.
Additionally you'll need to create the nameservers you've set up in DirectAdmin, at your registrar, where you've registered the domain under which you've set up the nameservers. You'll need to get support on how to do that from the registrar where you registered the domain name. Once that's done you'll need to edit the zone file where that domain's DNS is hosted, and add A records for each namserve. How to do that? That depends on where and how the DNS for that domain is set up.
Once that's done, for each domain you're hosting on your server change the assigned nameserver at the registrar where the domain was registered.
In DirectAdmin only a user can have it's own IP#. So first you get IP#s from your upstream provider and install them on the server, and allocate them to your ressellers as shown
here.
If you're not creating any resellers you can use the admin reseller account which is built in to DirectAdmin.
You need a separate user for each domain which needs it's own IP#, for information on creating users, you must first create a user package, look
here.
Once you've got a package, create the users you need for the package; look
here. Be sure to select a unique IP# from those you've added to the reseller account.
Once you've done that upload the files you need to each user's public_html directory. How you do that is covered in the documentation for the ftp program you're using.
Jeff