Some school experience is a pre-requisite of an application to the Northern Star ITT programme. This will strengthen your application and provide you with an invaluable insight into what teaching is really like and the role and responsibilities of a classroom teacher.
Contact schools who know you or local schools asking who the appropriate contact is. Be flexible. Schools are busy places and so might not always offer you exactly what you requested. Volunteering is a great option if your schedule permits.
Schools may require a DBS check as a matter of policy and you will need this as a trainee teacher. It would be wise to obtain this as soon as possible as they can take weeks to complete.