The strategy always needs to be, to choose topics with less competition for sites. A topic with less competition is easy to overcome than longing to overcome a huge competition.
To judge the competition you need to look at the sites currently ranked in Google for the chosen keyword and figure out how strong those sites are in the eyes of Google.
Google has its own method to judge and indicate the strength of a webpage, it is the Page rank(PR).
You must opt for a topic, whose results show sites and pages with a low PR( page rank), as it is easier to rank over these sites when compared to sites with a high PR.
Sites and pages with a PR0 and PR1 are the low PR sites and are very easy to outrank.
Sites and pages with PR2 are easy to out rank but not as easy as PR0 andPR1.
Sites and pages with PR3 can be outranked, but takes a lot more work over it.
Sites and pages with PR4 are strong sites and are difficult to overrank.
Sites and pages with PR5 are very difficult to over rank.
You can get the PR status displayed , by downloading the Google toolbar which contains the information on the page rank displayed on your toolbar itself.
You can download it here http://www.google.com/toolbar/ff/index.html
The above is the snapshot of the Google page to install the Google toolbar. Get it installed, and use the features displayed in the Google toolbar
As mentioned earlier, choose the topic with less competition, as there is more scope to get ranked in the first place of the first page of google when the competition is with sites with low page rank status. In case you opt aimlessly and intend to rank over the sites with a high PR, it is merely impossibly, perhaps may take a very long time. The duration , it takes to make money is proportional to your choice basing on competition you encounter.