I've learnt all my HTML knowledge online, search on Google, there's hundreds of pages devoted to teaching you HTML and ironing out problems. But the one site that i've got in my favourites (or 'favorites' as it says on my screen despite the fact English(UK) is my preferred language) and I use ALL the time is (drum roll):
The link above in particular (as opposed to the home page) i find most useful. Bookmark it now! All the links on that page open up another window that allows you to change html and see the result.