This is a summary of ideas from the book The 22 Immutable Laws of Marketing by Al Ries and Jack Trout. Text in normal is my paraphrasing of what the book says. Text in italic represents my personal comments. And remember: this is just a short summary and is not meant to replace the book, nothing beats reading the real thing. The book is short, buy and read it.
Law 1 (Law of Leadership)
Being first in the market is better than having a better product than a competition. Examples: we all remember who first flew over Atlantic or who was the first man on the moon but almost no-one knows who was the second. Heineken was the first imported beer in USA and still is No. 1 imported beer. Same for Miller Lite, first domestic light beer. Being first doesn't matter if the idea/product is not good.
I think it's better to say that being first gives one extremely big advantage over competition but doesn't guarantee the success. It's rather obvious that it doesn't matter that you're first to market if no-one needs your product or if your product is very bad. There are many examples from computer industry that disapprove this rule (i.e. first spreadsheet isn't the dominant spreadsheet, first word processor isn't the dominant word processor) so there are (many) cases showing that n-th product can overtake early leaders. But it's very hard and usually requires the leader to make huge mistakes.
Law 2 (Law of Category)
Given that it's very hard to gain leadership in a category where competition already exists, it's better to create a product in new category than trying to attack existing categories. Category doesn't have to be radically different, e.g. if there's dominant player in imported beer, one can become the first to import light beer. If one can't be the first to fly over Atlantic, one can still be the first woman to fly over Atlantic.
Law 3 (Law of Mind)
It's not important to be the first in the market but the first