Just like all things, as affiliate marketer you need to start somewhere, and often just making that initial move and beginning a "course of action" is the toughest part.  You know you want to make money, but if you just start with that goal in mind and haven't chosen a field of interest that truly is "your interest", you will have difficulty and your work will become just that - "work".  As a consequence, it will be more difficult to discipline and dedicate yourself to your chosen task.  

Therefore, explore your personal interests and select a field that you (because of your high interest) already know a lot about - expand on that knowledge and pass it along to your future customers.  

Once you have chosen your field of interest, a good place to start is by developing a blogging site in order to establish yourself as an authority on the subject.  In that way you are "laying tracks" to do all the things that you will need to be doing as webmaster on your own web site.  Once your web site is setup and running, you can link your blogs to your web site in order to increase traffic.  

Begin setting up your blog by carefully selecting a URL that: 1 - identifies what your blog is about, 2 - is keyword loaded and 3 - is uniquely "your" blog (e.g., Barry's Dog Training Depot, not - The Dog Training Depot).  

You can begin to development of your web site, without delay, while you build credibility through your blog (or blogs - why limit yourself?).  

Whether you have a "niche" market or a "monster" market, makes little difference; they both require the same basic course of action.  

Namely, you will need to add products to sell that are closely related to your web site content, add "feeder links" from your blogs and the blogs of others which connect directly to your web site.  In selecting products for your web site, be sure to not overload your site to the point that it becomes a "hodge-podge" of so much information that you leave your prospective customer in total confusion.   

Next, insure that your Meta tags (particularly the title, description and keyword tags) are correct and properly in place.  Meta tags are slowly losing their value to the search engines, but you working for all you can get; so at least add the three stated above.  A couple of words of caution - when setting up your keyword Meta tags, make sure that the keywords you use also appear in your article content and don't repeat the same words in your keyword phrases.  

In addition, develop an email marketing campaign which responds to the clicks on your web site and blogs.  This should be complete with - from 15-20 auto responder messages, which on a systematic basis, answer to your clicks on your web site and blogs, to "wind in" your prospective customers.  

As an adjunct to your email campaign you may wish to develop a (perhaps weekly) newsletter. Newsletters, take some time to service, and you therefore must make sure you are ready to take on the "burden" of commitment before you launch your first newsletter.  It is be far better to never start a newsletter at all, than to start one and quit a short time thereafter.  

It is important to realize that most likely it will take months to develop the web traffic you will need to fully monetize your web site and realize the kind of income that you desire.  Be persistent and blog as frequently as possible (at least once a week).  The more active your presence is, the faster the search engines will find you.  Stay motivated, believe in yourself, and your efforts will be rewarded.