That question is a common trap that people fall in to. When they ask "tell me about yourself", they want to see if you are the type of person that rambles on incoherently or continues talking until the interviewer nods off and drools on their desk.
Don't start from when you were an embryo in the womb and give excruciating details about your life... Just tell them where you grew up, what school(s) you attended and then a very brief work history. Ask them, once you finish if there is any detail that they wish you go in to a little more deeply.
Lastly, them asking you why you want to work there is not a silly question at all. This is your chance to shine! Give them a brief synopsis of what you know about the company and hopefully, how their product or service stacks up against a competitor, for example. Also, try to get to the source of their "pain", i.e. the reason the position is open and tell them how you can take away that pain.