What kind of certification do you need to become an air traffic controller?

I will answer this straight from the FAA website, its address is here:

https://www.faa.gov/jobs/career_fields/aviation_careers/

Minimum Requirements
To be an Air Traffic Control Specialist, you must:
Be a United States citizen
Start at the FAA Academy no later than your 31st birthday
Pass a medical examination
Pass a security investigation
Have three years of progressively responsible work experience, or a Bachelor's degree, or a combination of post-secondary education and work experience that totals three years.
Pass the FAA air traffic pre-employment tests
Speak English clearly enough to be understood over communications equipment

I hope this helps. You can find it all at the website that I posted in the beginning.

Minimum
What you might do is review your local community college classes and see if they offer Air Traffic Controller Courses and this will give you an idea of what is required. You can also go to your favorite internet search engine and type Air Traffic Controller Schools, etc and review the required courses.
You can also go to www.faa.gov and just type in Air Traffic Controller, but in any case the minimum requirements are:

•Be a United States citizen
•Be under the age of 31
•Pass a medical examination
•Pass a security investigation
•Pass the FAA air traffic pre-employment tests
•Speak English clearly enough to be understood over communications equipment
•Have three years of progressively responsible work experience, or a Bachelor's degree, or a combination of post-secondary education and work experience that totals three years
•Be willing to relocate to an FAA facility based on agency staffing needs