The very first thing is to figure out what do you like in yoga asana,
- Do you like easy asana? (Hatha yoga)
- Flowing Asana? (Vinyasa yoga)
- A discipline physical stamina practice? (Ashtanga yoga)
- With props (Iyenger yoga)
- Physical issue (Restorative yoga)
- Meditative and easy asana practice? (Shivananda yoga)
- In my opinion I feel that if you have never done yoga or any other physical exercise, you should go for 200 hour yoga teacher training, or Shivananda yoga ttc to gain basic knowledge of yoga philosophy, anatomy with easy asana practice.
If you know yoga asanas already and been doing for long time. I would suggest you should go for ashtanga or vinyasa 200 hour yoga teacher training.
If you have any physical issues, it is good that you go for iyenger 200 hour yoga teacher training.
After you decide what exactly you need training in, search for schools that offers the style you want to take up. Many schools also offer multi style. The real work of choosing the school starts.