Often the hardest part of a writer's job is learning how to sell themselves. This all starts with the query letter. It's a careful balance of professionalism, knowing your audience (the agents), enthusiasm for your work, and knowing how to put it all together. Learning how to mix all this and come out with an outstanding query letter is not easy.
Don't confuse enthusiasm for your work with overconfidence. Saying things like, 'there isn't another book like this on the market', or 'this will blow away Twilight', are red flags, not good points. If your book isn't like anything on the market then an agent is going to wonder if they'll even be able to sell it. You do want to express it's uniqueness but also touch on what it is similar too. Making a statement like the second one is arrogant and may come back to take a chunk out of your posterior. When doing comparisons be respectful of the author's work and make sure your accurate.
When it comes to the paragraph about yourself if you don't have any publishing credits, don't panic. Focus on what you've written, if there is a sequel available, or if you plan to write one. Here's some great advice from editor Chuck Sambuchino on the subject: http://www.guidetoliteraryagents.com/blog/What+Should+You+Write+In+The+Bio+Paragraph+Of+A+Query+Letter.aspx
Here's an example from Chuck of an excellent successful query letter: http://www.guidetoliteraryagents.com/blog/Successful+Queries+Agent+Michael+Bourret+And+Wake.aspx