how do I know what niche to start with?

Start with what you know, maybe a hobby of yours? Check they keywords out on google keyword tool and check for competitors ..

I have some reports for beginners here, totally free.
References :
http://smdn.us/reports

First of all,it is usually easier if you start with something you are really passionate about or very knowledgeable…..
You can check out this site,which you can get your own web page,and free text ads,and all kinds of business leads to get you started on a list,lots of support and help and a few other ways to make money as an affiliate…so if you would like to check it out, here is a link to the main page but remember there is a lot more than what is on the first page,and it’s free to join and free leads etc.
http://OrangeLeads.com?id=25737 Maybe there is something of interest for you here.
Best of Luck in Whatever you Choose!!
References :
Http://OrangeLeads.com?id=25737

Dear Neva,

Start with what you are most in to in the long term, something you’ve always been interested in.

A name for your business or website would be nice. I use my name, but there were more than one Diane Jensens online, so I chose three names, Diane Neill Jensen. There’s only one of me out there so far.

Set up a blog. I use GoDaddy, but there are others who host sites.

You will want the "paid" version of WordPress, go to your hosting site with your business name and get a one-year blog site with your own domain. Mine is http://DianeNeillJensen.com

Now, choose an affiliate site that offers what items that can go along with your favorite topic.

Start writing about your topic, get at least 10 informative articles on your site.

To promote, join Twitter and Facebook.

For finding people who might want to purchase your products, find niche markets within those venues where there are people actively participating with Twitter and Facebook, and start letting them know what you have, that you can answer their questions, etc.

How’s that for a start?

Good luck. Let me know what else you need to know.

Diane
References :
Twitter.com
Facebook.com
DianeNeillJensen.com
GoDaddy.com

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