You need to fill in a volunteer enquiries form this can be found here https://www.sja.org.uk/volunteering/register.asp
But it takes abit of time to get a reply, so your best bet is to ring up your local county headquarters. The list of all headquarters can be found here https://www.sja.org.uk/sja/about-us/st-john-ambulance-in-your-area.aspx
Good Luck, and I hope you enjoy being in St John Ambulance