Most (not all) start preparing their DCs sometime during Yr 5, depending on the amount of work you feel is needed, child's enthusiasm, number of different tests to cover etc etc - you have plenty of time yet.
Some use a paid tutor, others DIY. There are lots of materials available from this site if you decide to DIY and heaps of valuable advice from tutors & parents here as well. You could look through some of the discussions for your specific area. This is a useful thread if you want to DIY...viewtopic.php?f=12&t=4782