There are 3 basic things to consider when attempting to improve your website's rankings in search engines.
You need to make sure that you have quality related content on your site. Make sure that the words you use are words that your customers use. For example, if you sell 'motor insurance', then you should really be using the words 'car insurance' on your site, since this is what most customers will be searching for.
It's important to use your keywords in the main text on your site, but don't over do it! Writing the words 'beer beer beer beer beer' won't improve your rankings for the word beer... in fact, you will be penalised by the search engines for trying to 'spam' them.
The search engines are clever enough to pick up on 'spam' - don't fill your website with the words 'free','porn' or 'beer', if they aren't related to your site.
Search engines reward 'active' sites. You should try and add new content to your site on a regular basis (at least weekly). This indicates to search engines that your site is still 'live', and not an old disused site that hasn't been changed in years. It's reasonably easy to do this - set up a news page for example, or an articles section.
Search engines also reward 'popularity'. The more links on the internet that point to your site the better. The best way to do this is to approach the owners of sites related to yours, and ask them to add a link to your site from theirs. You can offer them a reciprocal link on your own site in exchange.
The quality of the link is important too. Provide the other site owners with your preferred link text, which should contain your keywords. For example:
Form a company online with @UK PLC
is much better than
External Link: @UK PLC
Note: the search engines are also watching for 'organic' growth in the number of links. Some companies offer 'link farms', where overnight you will suddenly get thousands of links to your own site. This behaviour is not good, and again can cause penalties and a drop in your search engine rankings.