To add an A record you need to first log into your domain's control panel, and look down to the DNS management area:

Click on "Manage your zone file".

You will now see a table presenting your zone file, and three links above it: 'simple', 'normal', and 'expert'. It is actually easier to use the 'expert' interface, so click on that. You will then see something like this (yours will likely be slightly different):

You will now need to add your desired A line.

Here is a model that you can follow:

www 10800 IN A 123.123.123.123

Simply replace "www" with the name of the subdomain you want, and "123.123.123.123" in the above example with the IP address that your provider gives you.

As a reminder "*" means "every subdomain", so if you have an "*" A record, then there is no need for "www" unless you want to override the ip address you gave with your * entry.

If you want just the domain, then you need the "@" record you see in the above example, or:

 @ 10800 IN A 123.123.123.123

Usually both the * and @ are used.

When you are done, click on the "Submit" button on the bottom of the page to continue.

All that remains now is to wait for the changed to take affect, so for this please wait at least 3 hours.