One of the most important decisions that you are going to make in the early part of your search engine optimization ventures has to do with the keywords that you are going to optimize your website for. Now, while every page of your website can optimize for different keywords and you can cover the whole list of pertinent keywords in that way, there are only a finite number of keywords that you can place on the front page of your website. Since that is going to be the page of the website that you get the most traffic to and therefore the page you want to optimize the best, the keywords that appear on that page are of the utmost importance.
Picking a Number
Because the on-page optimization factors of SEO work are quite easy to do and quite simple to pull off, it is technically quite possible to add a number of different keywords to the main page as being keywords that you want to optimize. However, this is not the best thing for you to do because of the fact that if you optimize for more keywords then the power of each keyword becomes less defined because you are going to have to dilute your content in order to fill the requirements for everything. In addition to that, if you optimize for more keywords that contain similar or the same words to each other as part of them, your content is going to sound very weird. Lastly, when you do the off page optimization later on, you are not going to want to have to do it for ten different keywords because that will create way more work for you than is necessary.
All in all, picking one primary keyword and two or three secondary keywords to optimize your main page for is the limit as to how high you want to go on your first try. Later on when you are better at writing optimized content and better at the whole SEO game in general, then you might want to take a look at doing it for more keywords. But for now, one primary and two secondary keywords should be good enough.
Once you have decided on the number of keywords, selection becomes a relatively easy process. Just pick keywords that have a relatively high ratio of searches to results and cross reference those with keywords that have a top-ranking website with a Google Page Rank of 4 or lower. Once you select a particular niche and do that research, you will end up with a list of candidate keywords that you could potentially use. Select one to be your primary keyword and two or three more to act as secondary keywords for the optimization of your main page.