My rule number one: if you don't know your competition right off the bat, don't simply assume they don't exist, look hard to find them ... (and if you end up finding them, ask yourself why they couldn't make it to a point that you would know about them without doing so much search, and how your product would be any different than theirs and same thing wont happen to you)

If you have direct competitors then it won't be too hard to tell who they are, you do some basic searches and you will find them.

But I think you should look at indirect competition as well... look at all those companies that if they intend to come in they can crush you over night (and I don't mean Google, Yahoo and the other big guys necessarily, I mean all those other companies that their business models allows them to do the same thing as you do, if they like to do so, but they never thought about it or had the intention to do it... sometimes they might have some strengths that would make them more successful than you.)

Also, I would keep in mind that if your product differentiates from the competition by just a few features you don't have a huge advantage

And if you ever intend to compete with any business that has any social aspects to it (i.e. web 2.0 businesses) never forget the network effects they could have just for being out there before you