DNS stands for Domain Name Service. It ties the domain name to an IP address.
For example: You would tie example.com with an A record to the IP address 10.0.0.101 and you would tie the sub-domain www to example.com with a CNAME (alias) record.
If you have a domain name, you would assign the IP address given to you by your host or domain forwarding service to the domain name using a A record.
example.com A 10.0.0.101
And you would assign the alias www to example.com.
www CNAME example.com
The TTL times should have a default value. This is the Time To Live value in seconds. This is a time value that tells any DNS cache how long to cache the value for. If there is no default value, then 3600 should suffice.
It is likely that your domain forwarding service will have the information you need such as IP address. It may be that another setting be set other than the ones I outlined. These are example settings for the most common use. Most free hosts do not allow a domain name be assigned to a sub-domain. But this does not mean that this cannot be done. You will likely need to ask for assistance on this matter from the domain forwarding service or possibly the host.