Most rallycross is sanctioned by the SCCA. I have participated in it since its origination in the U.S. Our region was the first in the US to be sanctioned by the SCCA. It helped that the national offices were in the Denver area at the time. I sat on the committee that set up the original rules which have been massaged over time. Basically there is accommodation for just about any street legal car (convertibles basically the only exception). Check the SCCA national web site and regional web sites for rules.
The Focus is a good car for rallycross use. I suggest using a tire with some tread. All season tires work well. Snow tires are even better but in some regions those may put you in a different class. Check both the national rules on your local regions amendments.
Basically all you need is a helmet (check rules for required certifications) entry fee, pass a tech inspection - (no loss items in or on car) good ball joints and tie rod ends etc. Here in colorado we get as many as 8 runs plus some fun runs. You get more for your money than autocross because there are usually fewer competitors. While the courses are on dirt or gravel the rules state that the course must be reasonably smooth to prevent damage and any possibility of rollover.
You'll have a lot of fun. If you in a good region a lot of Rally America boys will slow up with there rally prepared cars and get your juices flowing to want to join them in the future. You'll actually learn and develop handling skills quicker than the autocross people.