I think that you did a very good job on your first site. I know people that have taken classes and have not done as well. With that being said I have to say that getting a site to be cross browser compliant is a real pain in the rear, but it can be done. In time you will learn some of the ins and outs of what, how, and why certain browsers do things. IE6 is definitely a browser that makes me want to rip my hair out from time to time, but I have learned how to tame it after months of playing around trying to get different client's sites to render correctly in it.
The first thing I do when I build a site is I only view it in a standards compliant browser while I am building it, such as firefox. When I am done with the first page I then validate it and correct any issues. This is what was already suggested and you need to fix those errors, I doubt they will fix your issue, but you should still fix them.
The next thing to try and understand is how IE6 deals with DIV's. There are a couple of things to be mindful of.
First, Explorer calculates widths after the parent element is rendered so this can cause bugs when using percentages for column widths. You may want to make your sidebar an actual width like 200px or something like that.
Second, The content inside with padding might be larger than the main content box or larger than the sidebar. In firefox content inside a DIV will flow over the DIV without the DIV growing larger. This is not the case with IE6. If the data inside with padding is larger than the size of the div then the div will be made larger to accommodate the content. This will usually cause one of the div's to drop down below the other div instead of next to it.
Think of it this way. Say you have a total width of 1000px. 25% has been assigned to your sidebar and the rest is for your main content area. So 25% of 1000px is 250px. Now if the content inside the sidebar is greater than 250px wide (this includes padding) then it will spill over the sidebar. In firefox and IE7 and chrome/safari it shouldn't do anything but spill over the div's and should render fine. In IE6 it will make the sidebar bigger to accommodate the content so now it will be 300px instead of 250px, but your main content area has already been determined to be 750px, which now means you have 1050px worth of div tags but only 1000px of space to place it into so now it drops the main content box to the bottom.
If I were you I would set my sidebar to a fixed px width and set my right margin on my main content div to also be fixed px instead of 26% like you have it now. No real reason to have your sidebar in % in my eyes. Up to you though.