Certifications - How Do I Go About Them ?

I've been thinking of what to do with my holiday, yesterday while surfing, i came across a certification resource website. I took the A+ `and Linux+ tests and  for the few i was able to attempt due  to phcn, i passed them since they were things i know, especially the linux+ since i've been using linux now close to two years. Anyway my question is how do i register for the certification exam ? And any other useful information would be appreciated. Thanks