If what you have to say seems like a good and complete answer to the original question, then it is an answer (others will up- or down-vote it depending upon whether they agree with this assessment).
If what you have to say is about the question, or is a relevant detail that could be part of an answer, then it is a comment (e.g. the observation that *.SE is not a "forum" is true, but irrelevant to your question. Directing you to the "help" or "tour" page is relevant, but by itself isn't an answer.).
You should assume that eventually all comments will be deleted from the item.
If you think your response might be irrelevant, than you already know that it isn't an answer.
The original poster of the question can select the check-mark to indicate that one specific answer completely answers the question, and is actually useful. This selection is normally not done within 24 hours of asking, to give everyone in every time zone a chance to respond.
Note that these answers apply to all *.SE sites, not only askUbuntu.
Read the tour as you were linked. It answers all these questions you've stated.
As for converting answer to comment, that's a task that only moderators have teh button for so you can custom-flag your own post asking it to be converted to a comment and a moderator will eventually get to it.
You will be able to post comments once you get 50 reputation, so just contribute good questions and answers and you'll get that reputation fast.