I guess that my problem is quite common in vector drawing but I haven't found any answer yet : I'm currently creating the head of a character using lines that are combined to form the shape. The problem is that I can't colour it properly : the picture on the left is the result I get, and the picture on the right is what I would like to do.

I understand technicaly why I obtain this result, but is there a tool which can create a "closed" outline from my object ? If it's not the case, how can I close correctly my head shape, because the "join vertices" button doesn't work if one of the selecter vertice is not the end of a line ?
Thanks for your help.