First off, what is your product? If it's a downloadable info product you may consider ClickBank. I like their program for specific products.
2nd, I would only use a fully automated Affiliate software or online service. I prefer my own software because I am experienced enough to install and manage it.
program Karri suggest is considered excellent, especially for its low cost and many features. I like webscribble.com Web Affiliate. Also only $99 and they will install it for free. But I suggest you be fairly computer literate, understand the basics to installing CGI scripts in case something goes wrong and you don't want to pay for reinstallation.
Go here: http://www.webscribble.com/products/...liate_software
I use to travel around the country, as a Sr. consultant, and help setup MLM companies, and wrote a book on how to set one up ($150)
one or two tier depends on your needs. I don't know your product or market so I'll not comment on that, except to support jmjj215 comment that I normally prefer single tier over two, because you want to recruit Doers, not those that want to recruit many others to make them rich. Not all have that attitude, but I'll venture to state for the record, that the stats are in your favor that single tier recruits are going to produce much more than second level ones. Just pay the doers, is my motto.