you need a chain link remover tool , any good bike shop will sell you one, £ 7 , or around that , but check the crank first , one of the teet might be bent out of line, in or out make the chain come off put the bike up side down , and peddel , and if its bent you will see it were it comes off,
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix bicycles.