Whilst it's possible to learn the basics of yoga online, care must be taken to avoid injury!
Studying yoga online is great for inspiration and to discover new poses (and things you thought the human body just wasn't capable of!). You should never just try to replicate the poses and asana's that you find online on your own at home as you could seriously injure yourself.
The best thing to do is to find a private yoga teacher in your area and discuss with them your goals and maybe show them the things you found whilst searching for yoga online. They will be able to offer professional advice as to what you are capable of, and work with you to ensure you are performing the poses correctly.