I agree with Mike that the video can be a great place to start. When I first saw the video, I was amazed at how well it captured much of the teaching that happens at live seminars.
If you are going to start learning with the video, I would also say that there are certain other aspects of your practice that you should probably stress more than the average practitioner.
First, everything stems from you spirit, attitude, and "martial morality" (武德). The stronger you maintain these, the more you will learn and the more opportunities you will probably find to learn.
Second, the 24 and the 103 are quite similar, but also quite different. Be open to different details that may not seem to matter in the short run, but will matter in the long run.
Third, Taijiquan is easy to start, usually easy to practice, but hard to get really good at. If your goal is to become certified, you should, if at all possible, seek out live teaching. If regular teaching is problematic for your location, budget, or schedule, periodic teaching might be sufficient. Go to a seminar. Seek out an Association Center. Seek out a senior practitioner. Long-distance teacher-student relationship are actually not that rare. With the right chemistry between teacher and student, you can learn an enormous amount in a day or two that can give you material to practice for many months (or even years).
Fourth, strike a dynamic balance between being very patient with yourself and your progress and being very observant and very critical. Mirrors or video taping can help a lot.
Fifth, ask questions and read.
After viewing some clips, you can ask any questions here, and someone will help you out.
This is great advice as well.
As for ranking, the requirements are laid out on this site here
. Anyone who can perform the 103 from memory is potentially ready to enter the Ranking system and could contact a Center Director, Certified Instructor, or post here for instructions as to logistics. Much to my surprise, I have become a fan of ranking because of the focus and motivation it can give you to break through barriers and improve your practice significantly.