The best route is to have one or more of the following working for you assuming you are also physically capable of handling it:
1. A friend that produces stuff or somebody else you know with connections that are willing to cast you.
2. Starting out in gay porn like was said above.
3. Have enough money and know enough of what you’re doing to produce your own porn with you staring in it.
4. Starting out as a grunt in some production that needs massive number of men and somehow catch on with the people there.
5. Have a very attractive significant other that is willing to get into porn and then demand that she either work exclusively with you or at least get you work with her from time to time.
6. Have some form of at least minor notoriety among people that can be played off of as a porn gimmick or have some physical characteristic that will do the same.
7. Just have enough money to virtually bribe somebody in letting you star in a movie, or help fund some production.
8. Blackmail
Some of those work better than others.
Also, I hope you know that unlike what a lot of people out there probably think when they watch porn, being a pornstar, especially a male one, isn't all it's cracked up to be. To make any large amount of money you need to be working almost constantly, it's easy for the industry to kick you to the curb because their is always somebody waiting to replace you and there is the countless times your going to get an STD. A lot of them are also the lowest form of scum, and I wouldn’t want to work around them. I know I would never want to be that.