the first answer looks like a wall of text, but he has a lot of good points. i just didnt read it all because i have adhd.
heres my version, i hope its easier to read.
Step 1) save your money until you have upwards of 6000 dollars to freely spend. then,try to find any 4 wheel drive CAR. A good choice would be a Subaru Impreza, mitsubishi evo or Audi a4 quattro. do not sell your boneville, however.
step 2) try to get a second job, or a higher paying job, in order to have money to modify your car to rally specifications, or repairs if necessary.
3) try to get involved in any local rally community. You might even be able to get a job helping out another team. be sure to let them know you are interested in being a rally driver somewhere in the future as well. These people will have a wealth of information to get you on your way
4) practice. Dont start out by driving down icy mountain roads. you have to start slow, just like when you were little and started walking. Before doing anything, you will probably want to get better tires as well as a higher suspension. Start by driving in dusty flat land (maybe if your family goes on vacation to arizona or something, you can take your car along). once you think you know your cars limits, start doing more challenging things. There may be local dirt tracks you can pay to have some time on (theres where your higher paying job comes in)
5) be prepared. at some point in time you will break your car, whether its a flat tire or a tree in the front end. of all the money you make, you need to learn how to manage it, and to be entirely devoted into this. you may want to put 50% or more of your paycheck into the bank every week. you might be living in your parents house until you are 30, but if you really love what you are doing that wont matter.
By time you get this far, i think things will fall into place as they go. Well at least i know you wont be following the advice of someone on yahoo answers by step 5.
hope this helps