How do I make a hash.txt?
Sorry, this is a noobie question (yes, I know), but I've been having trouble with generating a hash.txt. Could someone explain this?
a hash.txt is just a plain textfile with one hash per line

or i didnt get you question
If you just want to test, you can take an example hash from:

Perhaps what you mean is that you have something that you want to crack, and need to have a properly formatted hash as input for Hashcat. John the ripper Jumbo package has a nice list of hash extract scripts. These scripts basically take some input and extract and properly format the hash that is printed as output. The output hash an then to be used by Hashcat as input:

In some cases making your own hash for testing is very straight forward as you can see from the first link of example hashes. E.g. creating an md5 hash for testing is as simple as running:
echo -n 'Password123' | md5sum
As Snoopy pointed out, the hash is simply a single hash per line in that case. In some other examples the hash is slightly more complex since it includes salt or some other extra information, but even then they often can be formatted rightly by hand or if you wish by using one of the hash extract scripts. Make sure that the output hash is always one per line.