I did TY last year, and I'd recommend that you start looking for work experience as soon as possible! It's a good idea to have a work placement sorted before school starts, even if you don't know the details of when you're working. If you do multiple work placements, make sure that you have your next one sorted at least a week or two before your current one finishes. Hope this helps!
I did ty this year and i'd say start looking around now. Make a cv ( plenty of examples online on what to write ) and start handing it around to places you'd be interested in working. Personally i didn't know that people were already sorted before the year even started! it was crazy. So i'd say if you already have a place in mind i'd go hand in my cv asap as there will be others applying for work experience.