You will need...
A spare outlet on your consumer unit - with 30ma RCD
SWA cable out to garage either underground or over depending on distance to garage
Another small consumer unit in garage with 2 mcbs one for sockets one for lights - look in Screwfix catalogue
Run garage circuits as normal from this depending on what you need
You will also need
Full part p installation certificate from either a registered competent electrician or if you DIY you will still need full certificate and BC approval at a cost of Â£100- 200
This is notifiable work and uncertified DIY is illegal - if something goes pear shaped later you are in deep trouble !!