WHO: I know a lot of people are relatively new to email compared to me and don't use it as often as I do either, so haven't learned some of the tricks I've learned. Don't be embarrassed about things you don't know about computers; we're all at some level of ignorance since there is so much to know and there are constantly new things to learn. I use a Mac, but I'll give directions that are useful to PC users also; hopefully they will apply to all email programs, though I know that might not be true.
WHEN: If you're forwarding a message that was sent to you (especially if it was addressed to a lot of people) or if you're replying to a very long message especially one being sent to an email list or a lot of individuals, it would be best to remove parts of the message that people don't need or want to read. I won't discuss copyright here, but you might consider if the sender intended it to be sent to others. A message like one with a lot of one-liner jokes or cute things kids say is an example of one which doesn't need to have the headers (the "from" line and all the addresses in the "to" line, date, etc.) sent and if you do so, you may be violating the privacy of the people whom it was previously sent to by sending their email addresses to others without their permission.
WHY: People become aggrevated when reading a lot of email messages (many get more than you do) and having to scroll through a lot of information that is unneccessary. Also, some people have old computers on slow internet speeds and can't easily down load very large message (some with a lot of quoting can be 10 - 15 KB in size). Selective quoting to replies also makes the answer more understandable since they may not remember the original message or not understand which part of the message you're referring to.
TO REMOVE UNWANTED INFORMATION FROM A FORWARDED MESSAGE OR REPLY: Click your cursor (the little line the mouse makes) just before the information you want to delete. Hold the mouse button down and drag over the text you want to delete - it will highlight the text. If you highlight too much, you can click the mouse again and the highlighting will go away and then you can start over again. If you highlight too little, you can delete sections at a time since beginning mouse users or people with motor control problems might have trouble highlighting exactly the right amount of text at a time. When the text you want to delete is highlighted, click either your DELETE button or your BACKSPACE button, depending on your keyboard.
A reply message to an email list, assuming everyone saw the message to which you are replying, should be cut to only the specific part that you are replying to. If there is only one part to which you're replying, most email list software allow you to highlight the part of the message to which you're replying by placing your cursor just before the information you want to quote and then click the mouse and hold it down while you drag across the text you want to quote. Then you can select "reply to all" and that part of the message should come up in the reply with the name of the person who wrote it and the date they sent it. Then you can click your cursor under or at the end of that quoted section and put in a couple of returns so that you leave a space between the quote and your response and then you can type in your message.
If you want to quote more than one section, start with the above method, then go back to the original message, highlight another section, copy it (usually the "copy" command is in the Edit menu or you can click the control or apple key and the "C" key at the same time) and then go to the reply message clicking your cursor into the message where you want it to go and pasting it there (click the control or apple key and the "V" key at the same time). You can then highlight this quoted section and find the menu selection that will allow you to show it as quoted (in Eudora it's in the Edit menu under "Text"). Remember to leave a space between your answer and the quoted message and make sure it doesn't show as quoted - there should be an "Unquote" selection, too.
When replying to a long message just to its sender, you don't need to have the name of the sender and date it was sent since they should be aware of it but if it's been a while since they wrote it, they might want enough of the message quoted so they can remember the context of it. Usually if you do just a "reply" you will get the entire message quoted and you can use the first method of deleting unwanted text and place your answer underneath it. Another method of making sure that the person you send it to sees the difference between your response and the text to which you're responding is to select a color and create your answer in colorful text (most, though not all, email programs can see color in their message and will realize that that indicates your response).
LONG CHAIN LETTER FORWARDING - First, try not to forward chain letters (see Chain Letters) and when doing so, carefully choose who you're sending it to. As stated above, remove headers and other unwanted text, but if there are a lot of quote marks that don't seem to remove by highlighting and using "Unquote" or if you want to reformat it and make it look more readable for your chosen friends you'd like to send it to, you can copy and paste the whole message into a word or text editing program that has a "find" and "replace" or "find/change" function. Then copy the characters that you'd like deleted throughout the message and paste it in the finder and then change/replace all. The first time you try this might not seem to save you any time over changing everything by hand, but after you learn the tricks, it can be a very quick way to clean up a message. If you don't want to spend the time to clean it up, it must not be important enough to send.
Further information about email usage can be found at the links below. (BTW, I didn't read all this to create the above tutorial, but I did try to point you to web sites that don't have pop-up windows.):