You are using 2010, and can add a background image at appearance > background in your dashboard. A few suggestions on backgrounds. The image file size should be under 100k in size or it will adversely affect your blog loading times and also could get your blog search engine ranking discounted since search engines now look at page loading times when determining ranking.
I prefer, whenever possible, to use small tiled backgrounds since the file sizes are typically in the 10 to 20k file size range. With tiled background images, the image only has to load once and is then repeated by the browser as needed.
There are a number of online HTML table generators that you can use that will generate the code for you and all you have to do is enter the parameters. Do a search for "HTML table generator" and you will get a number of hits.
As far as code for widgets and such, that is all dependent on what you want to do. There are limitations on the code you can use here and the restrictions are outlined in this support document: http://en.support.wordpress.com/code/ . Basically you just need to search for what you want to do. Anywhere you find "script" or "embed" or "object" in the code, it isn't going to work here. Although flash is not directly supported here, it can be embedded using the gigya shortcode in most cases.