If you are 17 years old, have passed your CBT you can climb on and drive away on any 125cc motorbike of your choice - with a crash helmet, with L plates, Insurance, MOT (bike older than 3 years) and tax. You have 2 years from passing your CBT to learn to do your practical riding test. You have to pass a theory test before you pass your practical bike test, but you can still ride with L plates if you haven't passed your theory. I would strongly encourage you to seek out local biker groups or Rospa to find learner courses. If you have no road experience you really are meat in the grinder of unforgiving road users and badly maintained roads. You need to learn to ride defensively. A local bike group will take you under their wing and offer nuggets of friendly advice and tips to help keep you safe. They might even have ride outs in the warmer weather. Good luck.
What to have at all times?
Your mobile phone for when you break down or run out of fuel. A small tool kit (wrench; multi end screw driver; tape) to repair things that drop off or fall off your bike, visor cleaner, a good lock/disc lock.
Keep your chain on the bike well oiled unless you get one with a shaft drive.