250 Would be a good bike to start on, Yammy WR's are good!

If you have not done any before buy yourself something not to expensive as you will fall off and break it lol.

Dont ride without appropriate safety gear (lived and learned), Boots, body armour, gloves, helmet etc,

Also don't spend lots of nice new shiny kit to ware as it will get muddy and ruined on your first time out haha

Deffo worth doing though can be real good fun we have some brilliant times through the winter! good luck!