Quite often you don't NEED any certificate. If you are a native speaker with a college degree this will already get you several job opportunities.
Alright, to start if you aren't a "native" speaker(From England, South Africa, The United States of America, New Zealand, Australia etc), you will want to take the Test of English as a Foreign Language (TOEFL). The TOEFL IBT (Internet Based Test) is the most known now as it includes speaking and writing, though the Paper Based Test is still accepted in some places. Be sure to check before you take the test to know what scores you need and what test is preferred. This is to prove your English level to your school. While it doesn't always substitute for being a native speaker (especially if it is a visa requirement), sometimes it does. You mentioned being an AP student, so I am assuming you are American and this doesn't apply to you. So that's the TOEFL (different than a TEFL)
Sometimes schools want more than just a native speaker; they want someone who has shown some commitment to teaching and has a certificate. That's when you'll see them ask for a TEFL (Teaching English as a Foreign Language) or a TESOL (Teaching English to Speakers of Other Languages).
Two of the more well known (and thus more valued) are the Cambridge Certificate in English Language Teaching to Adults (CELTA) and the Trinity College London Certificate in TESOL (Trinity). There are a lot of different courses out there, but the CELTA and Trinity are by far the two best known; that doesn't necessarily mean they are the best but they are reputable. They have hours of in class teaching and hands on time with the moderators. While the CELTA is technically focused on adults, I rarely see problems with this if you apply for younger learner jobs.
I think it is easiest to think of the whole thing and compare it to shoes.
The TEFL and TESOL certificates are shoes, and you need a pair to get where you are going. The CELTA and the Trinity are like Nike and Adidas; if you tell people you have them, they know you have "good" shoes.
Now there are a lot of online certificates; I wouldn't encourage these as many schools won't take them, BUT if you already have a job and you'll get a pay raise or you are just looking at jobs that take online ones then this is great.
CELTA has started to offer an online version, but it still requires you to to some in person time as well.
My advice: Look for a school you would want to work at that is hiring right now. It is too early to actually look for a job for you, but you can see what requirements they want. These are the requirements you should shoot to have. If they say, "minimum 100 hour certificate" then that is what you want. "No online certificates" then you need to do one in person. "TEFL with classroom observation" means that most online certificates are out. "CELTA or equivilant" tends to mean Trinity or CELTA.