I have installed phpbb3 premod and it's working fine. However I am very new to phpbb and am finding it difficult to manage the forum. I know these are very general and naive questions, but can you please help me out. Here are a few questions:-

1) How to post announcements?
2) How to make sticky threads like in vb?
3) How to add Google adsense & analytics code?
4) How to generate sitemap for forums?
5) How to show similar topics like this forum?
6) Does showing social bookmarking links help in SEO. If so how can I install them.

I have bought a new domain so I can play with the forums before it starts getting indexed, so please also suggest a few good addons which will increase user interest in forums. Thanks in advance.

And welcome

1) How to post announcements?

It's an option you see if you have enough right to when posting a topic

2) How to make sticky threads like in vb?

Same as announcements. Authorisation are quite specific, so one could only have the right to post sticky but no announcements.

3) How to add Google adsense & analytics code?

The tobi mod at phpBB.com is pretty good, I do not remember the url but should be easy to find with the author name. You may find more about it here : http://www.phpbb-seo.com/boards/search.php
=> Google Analytics and phpBB3

4) How to generate sitemap for forums?

We are on it : http://phpbb3.phpbb-seo.net/sitemap.php

5) How to show similar topics like this forum?

I'm not sure for phpBB3, but the mod could already have been released at phpBB.com or elsewhere, if you find one, please ask for a patch, I'm sure many would be interested.

6) Does showing social bookmarking links help in SEO. If so how can I install them.

Well indirectly yes, since it increases the probability that these service will be used to bookmark your site and thus provides with decent backlinks, but there is no guarantee since it all depend on how your users will actually use them.
Our phpBB2 simple social bookmark mod can be used with phpBB3 rather easily, but we'll release one for phpBB3.