you'll need a degree in psychology (probably a 2:1 but you may be accepted with a 2:2 and then you need at least a year's experience of working with young children - possibly as TA or nursery worker. Then you apply for the 3 year doctorate level training so if you have a degree in psychology already and the relevant experience then you are looking at a further 3 years. Starting from scratch would mean 7 years. This applies to UK.