The single most influential piece of real estate on your site is your title tag. Reaching back to the article on choosing a domain (link), by using keywords in your site name you can easily ensure that they will appear in your domain, many of your incoming links, and in your title tag, all of which is extremely important.
If you did not name your site with your keywords you will need to figure out what your prime keywords are and use them in your title tag as well. Such as in the slogan most often put after your site name.
As an example, if you ran a web development site called WD City you would want your title to read "WD City - Web Development Resources" or something similar. Naming your site with your keywords is best, but if you did not you must make absolute certain the keywords make it into your title tag. Failing to put your prime keywords in your title tag will significantly hinder your efforts to rank with that page for those keywords.
Further into your site I recommend a more specific to less specific approach. So you will want to write your title tags along the lines of "Page Title - Section Title - Site Title." Optionally leaving out section title if you either want to keep it shorter or if it isn't applicable. Again too, naming your pages or articles with your keywords makes it easier for you to include those keywords in your title tag, which in turn helps you rank for those keywords a great deal. Often people will want to name their articles using clever metaphors like what you may find in many offline magazines, but for SEO purposes you want to be very specific, literal, and keyword rich. So, in the realm of SEO, if you had an article about taking better landscape photographs, you would want to title it "How to Take Better Landscape Photographs" and not something "clever" but ambiguous like "Take Better Pics of the Hills & Sticks." Being literal in your titles and topics will help your search rankings flourish.
Title tag space is not infinite, search engines may stop reading it after a certain length, and regardless of that for each word within it all the other words are weighted less. Always think of percentages when doing this type of SEO. If your main keywords are 50% of your title tag, that is good. If they're merely 5% of the tag because you've stuffed it with many minor keywords and other unnecessary language, and that isn't good. There is no magic keyword density for any aspect of on-page SEO, so worrying about hitting an exact percentage is a fool's errand. However, you do want to be aware of your density, especially in high value areas like your title tag. You will want to weigh adding additional minor keywords against the affect they will have on the density of your main keywords.
The final issue you must remember when optimizing your title tag is that in addition to being the highest weighted bit of on-page search engine optimization, your title tag also forms the link people will be clicking on to reach your site from the search engines. As such you want to make sure it entices people to visit by avoiding the appearance of being spammy. This is also why we list the most specific information first in the title because that is likely what the person was searching for.
Meta Tags have a somewhat controversial past. In the 90's they were the main criteria for search engine rankings, however because they were so easily abused by webmasters they were all but dropped from use. However for years many refused to believe this and the SEO world was divided into two camps, those who believed meta tags were still a major factor, and those who didn't. Now though I think mostly everyone has finally come around to accept that meta tags are one of the least important aspects of optimizing your site.
However, being hardly important does not mean they aren't worth doing. The thing is you merely do not want to overanalyze or waste your time worrying about them.
The first meta tag is the META DESCRIPTION tag formed as follows:
This tag is actually somewhat important still as a search engine may use it to form an abstract for your site's SERP (search engine result page) listing if they cannot generate an on-the-fly abstract off of your visible content. So primarily you want this meta tag to accurately describe your site in a way that will encourage clicks from the search engines. Include your keywords of course, but your focus is more on clicks with this tag. Also, remember, that just because a search engine uses it for an abstract doesn't mean it provides any actual ranking benefit.
The second meta tag is the META KEYWORDS tag formed as follows:
This tag is used for providing a list of keyword suggestions to search engines with which they can rank your page, except most search engines ignore this tag nowadays. Still, you can easily make one in a few minutes so it is recommended you do so. Simply create comma delimited list of keywords and keyphrases and include them in the tag. Do not bother with repeating words, do not fret or stress about doing it well. Just get it done. The one piece of advice I have is to try to include less common words that your site is about but might not make it into your visible content with any regularity.