Sometimes it's hard to get to yoga class, and even putting in a DVD can be a bit time consuming. Start learning some simple yoga flows so you can do a full yoga routine by yourself without anyone directing you.
You can find many books with diagrams or learn some yoga routines in class or via DVD. A simple standing flow could include something like:
Warrior I - Warrior II - Reverse Warrior - Warrior II
To do both sides, you would connect this series with a "vinyasa." The vinyasa is simply:
Plank - Chaturanga - Upward Dog - Downward Dog
(This is where the term "vinyasa flow" comes from.)
So you would do the vinyasa between the left and right sides. Do the flow three times and then add in whatever seated or backbends you like. Voila! Home yoga!
I have been doing yoga for some time now and I feel that to really learn the postures correctly, it is always good to go to a certified teacher. Once you know it well, then you can practice at home. Yoga looks easy, but there are many parts to each movement and for getting maximum benefit, each posture must be done correctly - and this is where a good teacher can really help out.
A teacher (even certified) is in no way a sure-fire way to learn a pose correctly. As I've said here elsewhere, I've been to plenty of yoga classes where people are misaligned and the teacher does not adjust them. There are often too many students or too much going on. Once you know the basics, you aren't going to harm yourself by doing Warrior I less than perfectly - as long as you listen and pay attention to your body. We should not be made to feel "helpless" by the idea of an expert. Watching a video, reading good yoga books, and paying attention to yourself can be just as, if not more effective, than going to a class.