So we have Forums here on CSS-Tricks which are a lovely place to get and give help about web design. Now that I've posted almost 500 posts there, I think I have a pretty good feel for what kind of questions make for a good thread and are the most likely to be helpful. Here are some pointers:
1. Provide a link to a live example
This sometimes isn't possible, but if there is any way you can, please do. It is ten times easier to load up the site and diagnose a problem if we can see it in our own browsers.
This doesn't mean push code out live on real website that you know is causing problems just so we can see it and try to help. Set up a test page that you can link to, in a sub directory of the site.
A great way to do this is to make a Pen on CodePen.
2. Provide code samples
Code always helps, BUT, use your best discretion when deciding what to paste in. A lot of times people will post their entire HTML and CSS file and there is so much code it's hard to even scroll through and find the particular bits causing problems. If you have absolutely no idea, that's fine, but it's typical you probably have some idea of the problem area. Post that.
Also, remember to wrap your code in
tags, which help make the code far more readable in the forums.
3. Explanatory illustration / screenshot
If you find it hard to explain what you need in words, sometimes an illustration can do. Screenshots are also very valuable. You can include images that are hosted elsewhere on the boards with the [img] [/img] tag, as well as upload images directly.
4. Exact details
Nobody is more familiar with your problem right now than you are, so you are the best qualified to explain it. Don't be vague. "It's not working right in IE!!" doesn't help much. "When you mouse over the last menu item in IE 6, the submenu pops down but it looks 3 pixels to the right." is far more helpful.
Lots of questions involve cross browser issues, so make sure to include what browser you are seeing troubles in (and which you aren't) if possible.
5. What you tried / What you thought would happen
Chances are if you are posting in the forum, you've already been doing some troubleshooting. Let us know what you tried, and what happened.