How do you start practising Wicca?

You begin by finding a Wiccan circle. Speak with Wiccans in your area and ask to observe a ritual. See for youself if it is what you want and join if you are invited. Do some serious reading about Wicca. Find out what Wicca really is not what you have been told by those frightened or feeling somehow threatened by Wiccans. In the area you live there may be a Metaphysical/New Age/ Pagan/Alternate Religion Book shop... Most of these have a notice board where people involved , or looking to be involved with paganism can communicate with one another... Check these out. Also, most universities and colleges have similar noticeboards. Look into these, be prepared to get to know the people before you get an invitation to "join". Do some reading and research on your own and find a path you are comfortable with. Not all Pagans are Wiccan.