Employers are looking for some evidence that you can be successful in the job. In the past they would have been looking for a college degree. Many employers are becoming more flexible in what they will consider, but there has to be something more concrete than "I believe I already have enough knowledge..."
The best you can do is build something that shows you actually have the skills. You can do this in a number of ways:
- make significant contributions to an open source project
- build out a personal website as a portfolio
- build some other prototype system that shows you have built something of complexity that demonstrates your skills
- freelancing for cheap and non-profit work (usually on a volunteer basis) could help build a portfolio (thanks @panoptical)
Any of these things can get you in the door for an interview where you will have the opportunity to explain to them how good your skills are.