I decided I wanted to teach maths because I believe the skills we learn in maths are the most important skills we learn, and seeing so many people leave school without being able to add numbers is shocking.
I did my undergrad in Maths at York as well, and I felt like the teaching and support I received from the lecturers and seminar leaders was next to none.
Teaching my first lesson and seeing that in the space of an hour, pupils had learnt something that they didn't know before was hugely rewarding.
Get as much experience as possible. Take part in YSIS as they are great at supporting you and helping you to get the most out of your experience.