If you are interested in teaching yoga a good place to start is at the web site of the Yoga Alliance. There are many schools of yoga which offer certification. The Alliance does not, but it maintains a list of schools which provide training that meet certain basic requirements. It also keeps track of teachers: what training they have had, how many hours they have taught, and so on.
There are several levels. Baseline is the RYT 200. They have had a basic 200 hour yoga training course from an accredited school. The top step is an ERYT 500, who has significant training, and at least 4 years teaching experience as well as training. Of course there are some senior teachers who trained in the old school style, of training with a specific teacher, and who are so well known and respected they may not have gone to the trouble of getting on the register. Most teachers going into yoga now do go on the register.
There are some yoga like paths, that require less rigorous (or long) training. Keep in mind what you want to do. If you want to teach in a yoga studio the requirements may be different than in a health club, for example. But you do not want to go to teach and be asked questions about your lineage, or the science of pranayama, or what a drishti is, and not know.
You may be able to do teacher training without a big residential commitment. There is a real explosion of training programs. But also consider that means there are tons of teachers coming out of those programs…. how well known is the program you do, where do their graduates teach? I do a lot of training at Kripalu. It is within a few hours drive and I can get world class teachers (but I have taught quite a while so my needs will be different than someone building a practice). Where do you live? The second step might be to search programs near you for cost and schedule of training.
Also, what style of yoga do you wish to teach? There are huge differences. Some are very athletic, some are more therapeutic, some more gentle. The training program for Iyangar teachers is very very rigroous, but produces some of the best teachers out there. Vinyasa style is very popular, and there are quite a few varieties of that practice. If you do not have a regular practice yet I would suggest taking as many different places as you can, and finding a place and a few teachers with whom you connect.
Good luck, and feel free to message me to my page if you have specific questions as you look further.