Cyclocross (also known as CX) is one of the most fast paced and accessible disciplines in all of cycling. The discipline combines many of the skills used in standard road and mountain biking with an added emphasis being placed on bike handling skills as the riders navigate the sometimes bumpy and muddy terrain and physical fitness.
There are bikes specifically designed for cyclocross which are essentially a cross between mountain and road bikes.
For beginners however, a normal mountain bike will be fine as they are suitable for the off-road terrain.
It is advised to wear warm weatherproof clothing if possible as the racing takes place through the winter and will likely get muddy.
Helmets are compulsory for cyclocross and if you have a regular road helmet that will be ideal. So, there is no need to buy a new one.