Simply remove all the decorations and tinsel from the tree, remove it from it's pot or stand, then take it to a collection point. This is usually the boundary of your property or a designated communal point.
You may receive a leaflet from your council, find an information sticker on your bin or see an advert in your local paper detailing when the 'special collection' will take place. If you don't come across any of these, why not contact your council and ask if they are collecting them in your area.