Hello K. O.,
your first step is to pick an organization through which to become certified, and you will have many to choose from. Here on this website are many organizations listed by category where you have a good comparison. Here is the link http://www.ideafit.com/fitness-certifications. If you have a specific place in mind where you think of teaching, I would find out whether they have a preference for one certification over another. Not all certifications are created equal, and some carry greater standing in the industry than others.
IF your intention is to teach group fitness, I would start with a general group fitness certification like the one from ACE before moving into a specialty. However, there are organizations that specialize such as the Senior Fitness Association (SFA), the Aquatic Exercise Association (AEA) or Yoga Alliance (and there are plenty more).
Some certifications require that you attend a workshop, others (such as ACE) can be done entirely at home and online with a test at a proctored site. This may also be a factor in which certification you choose because travel expenses can make a certification significantly more expensive.
Hope this helps.