It is likely to take some time and considerable effort to develop any reasonable level of proficiency in piano tuning. For this reason, if you are a pianist, it's best not to practice on your own instrument, because it is likely to be unusuable until you have spent several months learning and have developed some skills. A second piano for tuning practice is thus a good way to go. If you spend, say, $1000 on a piano to use for studying tuning, you will also have a chance to practice basic regulation, voicing and repair skills that any tuner needs to know (if action parts are not working, you cannot tune a piano and must fix it first), without putting your primary instrument out of commission.
The only certification is passing the Piano Technicians Guild Registerd Piano Technician exam, which includes tuning, repair and regulation skills.
Piano tuning is hard work and very demanding of mental and physical energy. You will discover this after spending a whole eight hour working day attempting your first tuning. After which, your piano will sound miserable. I don't want to discourage you, but simply to warn you to be prepared to commit some serious time and effort.
The best place to learn about piano tuning is from the Piano Technicans Guild. There is a PTG National Convention next month, from which you could learn a great deal in a very few days for an investment of perhaps $1000.