I could have gone on length on answering your query. I am still astonished over the insignificance given to external links in JazzMoney's answer. At the first part, one need's to understand the meaning of page rank.
In short, here's an example. There's an election and several candidates have stood up. Its natural that the guy with the most votes wins. Analogously, whenever a external page links to your website, it casts a vote for your website. So, when you search for a relevant keyword for your pages, the results come as per the no of links that you have collected.
To get more links, you can ask some of your friends who have website to put link to your website. The more the links, the higher is your rank in google results. An alternative is to start your own blog on 3-4 websites, and in each blog, put a link to your website. Indeed, if your site has quality content, mpressed visitors start linking you, specially the blogging community.
Another important part is that who links you. For example, if you get a link from a website which already has a high page rank, you automatically get a better rank. For example, if you get a link from microsoft.com or cnet.com, which have a highest page ranks, your rank shoots up automatically.
Google rank is composed of many other factors, but the only factor it misses out is the popularity by visitors who go to the links directly. Yahoo does a good job by keeping that into account. If you carefully see the link that are seen in the Yahoo search results, a similar vote is also casted by clicking on the link in the search results. It could be possible that your website has huge no. of visitors who come through searches, but small number of links. Then its Yahoo rank would be high (more accurate), but google rank would be low. There have been many such interesting cases, so don't just consider google as your only target. Why google does not keep this 'click popularity' feature is because it takes a certain delay of time( for registering of vote on Yahoo and forwarding to the actual link), which Google does not want to take.
Things You Should Never Do In Life:
Never try to promote your site commercially from spamsters. Some of their sites are banned from google database. And if they give a link to your website, your page rank drops down drastically, and even might get removed from the search results. Also, do not link to their websites.