1. I was 15 when I first started volunteering.
2. I started off as a Journalist for First News, which was a voluntary position I applied for online. The NSPCC then contacted me offering me to apply for a voluntary position within their charity, and I agreed. This was when I really started volunteering for a multitude of charities across the UK.
3/4. Yes, I do voluntary work in my free time whilst being a Year 12 student in Sixth Form. I always prioritise my studies, but when I have any free time, I choose to spend it by volunteering. Most voluntary positions are flexible, so you choose what opportunities you want to volunteer towards - whether you want to do a couple, or loads! As for me, I manage my time by doing my volunteering in my free periods, over some weekends, or (primarily) during my holidays. Sometimes there are voluntary opportunities in person, sometimes they are online, which often makes it a lot easier for me to have access to more voluntary opportunities when I might not have the time to be there in person. I would say take on as much as you realistically can handle, as if you do too much, you may feel overwhelmed. Just do as much time as you can dedicate without it overwhelming you. Charities are often flexible about voluntary hours, so this isn't a worry for me.
5. Usually a charity would email me inviting me to join their campaign, such as helping formulate it, promote it, and more. If charities are looking for volunteers for campaigns, they often tend to advertise this publicly online.