How Google actually calculates your PR is a well guarded secret. Many till you what they think it is but no one can be absolutely sure (except Google).
We do know however that if you follow certain SEO best practices your Google PR will increase. Just is patient because this will take time. Subject to how diligent you apply these processes, the time period my be less for some than others in achieving a high PR score form Google. At worst you will notice an improvement in your Google PR score leading to a higher search engine ranking in the search results.
To start you have to have a website that contains pages relevant to your area of interest. Do not confuse search engines by mixing pages within your site where some are about women's cosmetic products and other pages are about home hardware tools. The search engines simply do not see the relevancy of the two products and will consider your site less relevant. If you do sell both these products then set up separate websites for each product.
Your PR score is Google's value of your site's reputation. The more reputable the site appears, then the higher the Google PR score.
So how is that reflected? For beginners, having a domain with history (reputable) is a good start. If buy someone else's domain be sure it has never been black listed. A good start is to check out its history. A handy tool for providing such history can be found at WAYBACKMACHINE which holds snapshots of the domain contents so you can see what the domain has been used for in the past.
In order to achieve a high Google PR score you need traffic and you need referrals (by way of backlines) to your site. So what is a good PR score? As I indicated earlier, if as a SME business you can achieve a PR score of 3 be content however you should aim for a Google PR score of 4 or greater.
Traffic and referrals area bit like "Which came first, the chicken or the egg?". Generally most sites create traffic to themselves through backlines they have from other websites. You may however launch some form of viral marketing campaign or run regular newsletters to create early interest and traffic in your new site. These same techniques should be used on a regular basis so as to maintain a steady stream of traffic along with some other initiatives I will outline here.
Building backlines is probably the most powerful tool you can use. What you do need to be mindful of is that the backlines themselves are from reputable websites and that they use your keywords in the text link to your site. In doing so you must however be sure that you give people good reason to link to your site. It must provide rich content that is relevant to your area of interest. You may wish to read my article about the notion of Content is King
Some say that you should only seek backlines from websites with a high Google PR score them. I am not as entirely convinced as much of the evidence I am hearing of would suggest that the effort and reward of chasing such sites may not be as advantageous as chasing lower ranked websites. Backlines from high PR ranked sites are harder to get, they owners are more particular about whom they will link to and they are not always as keen to fill their own site with backlines. Lower ranked PR sites are often easier to get and require less effort in follow through. At the end of the day, 10 backlines from a site with a PR score of 1 may be better than 2 backlines from a site with a PR score of 4.
The other point that I will make in closing is you don't have to chase company websites. There are many other places in which you can place some content with a link to your website. You should not overlook the use of social bookmarking site, social and business networks, business directories, blogs and forums(Check the article "How link text can make a difference in your rankings"). All of these offer you the ability to place backlines to your site. Some will be more effective than others and some will be totally ineffective. Test them out before you try.