This guide cannot teach you everything about Linux. There just isn't enough space. It is almost inevitable that at some point you will find something you need to do, that isn't covered in this (or any other) document at the LDP.
One of the nicest things about Linux is the large number of forums devoted to it. There are forums relating to almost all facets of Linux ranging from newbie FAQs to in depth kernel development issues. To receive the most from them, there are a few things you can do.
The first thing to do is to find an appropriate forum. There are many newsgroups and mailing lists devoted to Linux, so try to find and use the one which most closely matches your question. For example, there isn't much point in you asking a question about sendmail in a forum devoted to Linux kernel development. At best the people there will think you are stupid and you will get few responses, at worst you may receive lots of highly insulting replies (flames). A quick look through the newsgroups available finds comp.mail.sendmail, which looks like an appropriate place to ask a sendmail question. Your news client probably has a list of the newsgroups available to you, but if not then a full list of newsgroups is available at http://groups.google.com/groups?group=*.
Now that you have found your appropriate forum, you may think you are ready to post your question. Stop. You aren't ready yet. Have you already looked for the answer yourself? There are a huge number of HOWTOs and FAQs available, if any of them relate to the thing you are having a problem with then read them first. Even if they don't contain the answer to your problem, what they will do is give you a better understanding of the subject area, and that understanding will allow you to ask a more informed and sensible question. There are also archives of newsgroups and mailing lists and it is entirely possible that your question has been asked and answered previously. http://www.google.com or a similar search engine should be something you try before posting a question.
Okay, you have found your appropriate forum, you have read the relevant HOWTOs and FAQs, you have searched the web, but you still have not found the answer you need. Now you can start writing your post. It is always a good idea to make it clear that you already have read up on the subject by saying something like ``I have read the Winmodem-HOWTO and the PPP FAQ, but neither contained what I was looking for, searching for `Winmodem Linux PPP Setup' on google didn't return anything of use either''. This shows you to be someone who is willing to make an effort rather than a lazy idiot who requires spoonfeeding. The former is likely to receive help if anyone knows the answer, the latter is likely to meet with either stony silence or outright derision.
Write in clear, grammatical and correctly spelt English. This is incredibly important. It marks you as a precise and considered thinker. There are no such words as ``u'' or ``b4.'' Try to make yourself look like an educated and intelligent person rather than an idiot. It will help. I promise.
Similarly do not type in all capitals LIKE THIS. That is considered shouting and looks very rude.
Provide clear details stating what the problem is and what you have already tried to do to fix it. A question like ``My linux has stopped working, what can I do?'' is totally useless. Where has it stopped working? In what way has it stopped working? You need to be as precise as possible. There are limits however. Try not to include irrelevant information either. If you are having problems with your mail client it is unlikely that a dump of your kernel boot log (dmesg) would be of help.
Don't ask for replies by private email. The point of most Linux forums is that everybody can learn something from each other. Asking for private replies simply removes value from the newsgroup or mailing list.
Do not post in HTML. Many Linux users have mail clients which can't easily read HTML email. Whilst with some effort, they can read HTML email, they usually don't. If you send them HTML mail it often gets deleted unread. Send plain text emails, they will reach a wider audience that way.
After your problem has been solved, post a short followup explaining what the problem was and how you solved it. People will appreciate this as it not only gives a sense of closure about the problem but also helps the next time someone has a similar question. When they look at the archives of the newsgroup or mailing list, they will see you had the same problem, the discussion that followed your question and your final solution.
This short guide is simply a paraphrase and summary of the excellent (and more detailed) document ``How To Ask Questions The Smart Way'' by Eric S Raymond. http://www.catb.org/~esr/faqs/smart-questions.html. It is recommend that you read it before you post anything. It will help you formulate your question to maximize your chances of getting the answer you are looking for.