There are no instructions on how to use the forum, most of us learn by trial and error. There is a sticky laying out some rules for using the forum
https://www.fishforums.net/threads/forum-signature-other-rules.422935/ but this lays down what is expected of users rather than how to use the forum.
To answer some of your questions:
To quote part of someone's post, highlight the part you want to quote and a black box will appear under it. Click on Reply and a reply box will will appear with the higlighted sentence at the beginning. Just type the rest of what you want to say.
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Be careful not to touch the [ ] that appear round the words at the beginning and end of the quote or your reply will be inside the quote too.
A sticky topic is a topic that has some important content. As more threads are started, the older ones drop to page 2, then page 3 etc. Sticky posts remain at the top of the first page so they can easily be found and referred to. Often stickies have been locked so that new posts cannot be made in the thread; these have a padlock symbol in the title line.
Posting in old posts is a bit vague. If you have a problem, start a new thread rather than adding to an old thread with with a similar problem. It is better not to add a reply to a thread where the last reply is more than a year ago as the person who started the thread may well no longer use the forum, start a new thread instead
Any more questions, just ask