I would recommend that you try a few themes and just play with them for now. Stick to the ones in the WordPress repository. There's no need to pay for themes. All "Premium" themes are not premium at all.
A lot of themes come with excerpts on the home page and then read more to the full article.
The default commenting is usually perfectly fine for most.
With regards to user manager, I'm not sure which plugin is best. It depends what you want to do.
For forum, check out buddypress.
With regards to your second question, check out the article I wrote yesterday - http://wprealm.com/blog/making-the-transition-from-html-to-wordpress-where-do-you-begin/
I would also encourage you to look at making child theme and just take the default themes and look at the files. The best way to learn is to see how others have created themes.