Trouble with google sign in on my used G1?

My dad gave me his friends used G1. it's almost new, but i can't do any of the app stuff because he signed into google with his own account. I'm not really sure how all this works so i might be using the wrong lingo, but i want to sign into/register with google with my own gmail account. but the only option it gives me is to put it my dads friends pword. help?