I agree with count_drac, volunteering is a great way to get experience and something to talk about in your personal statement and at interview. It might be worth looking at volunteering in care homes or with people with learning or physical disabilities as all of the experience and skills are transferable. I have heard of midwifery applicants working/volunteering in care homes, so any experience within healthcare is an advantage and a great way to learn more.
I'd recommend looking at what the entry requirements are for the universities you would be applying to, so you know you are working towards them now, as you said you are at college at the moment.
NHS careers and the RCN website are great for advice about nursing careers, so you can find out more about mental health nursing and what it's like to work as a mental health nurse.
Hope this helps!