Owenw16

I'm completely new to motorbikes and recently acquired my CBT. I've purchased a Yamaha MT125 and on my 1st ride today quickly had issues with the pressure of the tyres. They were at 15 PSI, the max according to the tyre is 38. Can someone direct me to guidance based on size of bike etc? I've since set both to 28 however really hoping someone can advise what would be a decent setting.