The best place to start, is Netscape 2.0. Netscape 2.0 has a built-in debugging feature which will catch purely syntactical errors.
Go to the File menu and choose Open File. When your page has loaded, go to the View menu and choose Source.... A window will pop up showing your page's source code, color-coded. Anything that it sees as obviously wrong will be blinking. It won't catch everything, but it does catch all those unpaired quotes and brackets. Use any editor to fix the problems it points out.

Now, Reload. If that didn't fix what was wrong, note where on the page the problem is. View the source again and try to find the piece of code that is not working. If there's any tag you're not sure about, look it up. If you can't find anything wrong, try looking higher on the page. Sometimes a misused tag can affect things lower down on the page.

One really useful trick for catching mistakes is to spread everything out a little. Go through and double-space all of your code. (It won't change the way your page looks. HTML doesn't recognize multiple consecutive whitespace characters.) Also, break up long lines, especially if they are too big to fit in your editor's window.

Here are some common problems and their possible causes and/or solutions.

My link doesn't work.

First thing to do is visit the site you're linking to. Sometimes sites change URLs without warning. If the site is indeed there, click on the Locator (below the back, forward, etc. buttons) and copy the address there. (CTRL-C in Windows, or Edit, Copy) Go back to your editor and paste the URL into the link.(CTRL-V in Windows, or Edit, Paste) Now you are sure to have the correct URL.
If that still doesn't work, check your HREF. Make sure you closed your quotes. Make sure you closed your brackets. Make sure you included the </A>. And, make sure that you spelled HREF correctly.

My image won't load.

The most common cause of images not loading is either the images aren't where they're supposed to be, or the link is incorrect.
Did you upload the images to the right place? Check it. Try entering the address of the image in the Locator. Did your image come up? If you got an error message, odds are the image is not located where you think it is. If it did come up, try checking your IMG statement. Make sure you closed your quotes. Make sure you closed your brackets. And, make sure that you spelled IMG and SRC correctly.
If your image partially loads then quits, the copy on the server is errored. Try uploading it again, and make sure when you FTP that you're using Binary, not ASCII transfer mode.