In selecting keywords, try to choose those keywords that have high search volume but are less competitive.
Hmmm competition is the wrong metric to look at in my opinion. You can have a hell of a lot of results for a keyword, but that doesn't necessarily mean it will be difficult to rank for. Make sure you use something like "SEO for firefox" to get the number of links for the first 3 results on your search term. This will give you a much better idea of difficult it will be to rank for a term.
first do competitors analysis,.. then do keyword research on that. finally take some green keywords,..