Why should i join a Union?  

To protect and advance your pay, terms and conditions of employment and conditions at work. If you don't, who will?

Which Union should I join?

Ask around.. is there already a union at work? If there is, find out who the the union representative or shop steward is and tell them that you want to join. If there is not a union then either contact Worcester Trades Union Council or look at the descriptions of the national unions at the TUC website. http://www.tuc.org.uk/tuc/unions_main.cfm