Validation is a useful tool — invalid code may break the site in some browsers (so you need to test at least in the big 4 browsers currently in use: IE6 and IE7, Firefox and Safari), and may also cause issues for non-human readers (search engines, text readers for the visually impaired, etc.). Validity also helps ensure that your site will probably work in new versions of browsers as they come out in the future. You should therefore try to make your site as valid as you possibly can.
That said, it’s worth learning which errors are minor and which are really important.
Things like failing to provide “alt” text for your images are fairly minor — although conforming to this rule helps search engines and people who suppress images when surfing (low bandwidth, visually impaired, etc.).
Things like failing to close tags, having invalidly nested code, or using deprecated tags are major and may cause your site to break in some browsers, or in future browsers. You should definitely fix those.
(Others will disagree with me here and say that you should always make sure that your site has perfectly valid code.)
If you’re going to be working on your own site, you will save a lot of headaches by learning basic HTML (fairly easy) and CSS (harder — especially “positioning”, i.e. layout).
There are lots and lots of HTML/XHTML guides online — just make sure that your guide is fairly recent, because HTML has evolved over the years, with many tags becoming deprecated, and certain practices changing (e.g. use lowercase for tags; self-closing tags, etc.)
A great site to learn CSS is Westciv’s Complete CSS Guide — they offer paid tutorials, but the free stuff they have online is more than adequate to give you a thorough grounding.
Also, check out the Firefox extensions I mentioned above. And there are many good editors, both free and commercial, which will highlight your code in colors when you edit it, making it easier to see the structure, and many of which can validate your code as you work on it.
PS Do NOT use Microsoft FrontPage or Microsoft Word to create pages, and be wary of using any WYSIWYG tools to design web pages — unless you are can check and fix the code such tools generate (visual editors are a useful tool, but not good to rely on).