How do I start optimising my website?
Once you have decided which key phrases you want to optimise for, and which page you want to optimise for each phrase, make sure that phrase is included in the page <TITLE> tag, and worked naturally several time into the text of the page.

Then, you need to build up a network of links from other sites to yours. You can start by submitting your URL to reputable directory sites, and arranging reciprocal links with good quality sites that are complementary to your own.

How important is the keywords tag?
The Keywords meta tag was important for a while, but over the years it was increasingly misused. As a result, most search engines now pay it very little attention, and Google ignores it completely.

Is it worth posting links to my site on social networking and bookmarking sites?
Absolutely. But make the links informative, not purely self promotional. And make sure they are in content that is Public and not restricted to your Friends or Followers, or the search engines will not be able to find them.