1) I have noticed that using post tags do help you in SEO. If you follow matt cutts, you would know that he always suggests everyone to use features that provide a better user experience.
So, if you have a post and use the post tags that would help users find other related posts easily ( better user experience ) and also helps the search engines better categorize the content on the page.
2) I haven't really provided a description for each category on any of my blogs but I guess it might help. Write 3-4 lines with targeted keywords. It won't have a negative effect, thats for sure.
Don't understand what you mean in the 3rd question, so would leave it for someone else to answer.
By the way, I would recommend you to use canonical urls (www.example.com ) and also set that in the webmasters panel, if you are using it.
1) i guess using tags helps! install the plugin auto-tags, set it 5-5 (you will see what i mean when it's up) and you're good. you don't have to worry about the tags anymore, and it kinda helps you with the seo.
2) here's what i do. i use my main keyword in every 2nd title and use the keyword in every single description. also, you can add other keywords to the text! I always stay at minimum 50 words for each post. if you ask me, a 100-110 description is enough and it will totally boost your seo.
also, make sure to add the tags widget to the main page, since it will totally boost your rankings. <- my own experience.
make sure to add your site/sitemap to google webmasters tool
3) never used forums before, but if you ask me, i would let google find it!
Thanks for these great tips. Ill look into it and test it out. What I mean by 3rd questions is that if I have a forum on the same website will that do negative on my SEO as a wordpress tutorial website? Will google rank me 50% for Wordpress and 50% for forum? So I have to compete in both fields?