How do I start a new lawn in clay ground?

I just moved into a new contrusction home, and I ruined the seed planted in the backyard while working on my patio. The ground is mainly clay. It is very difficult to dig through, rake, etc. There is crab grass every so often in patches, but that's about it it. I purchased top soil that I plan on spreading out over the clay once I am able to treat the ground first. My questions are as follows: Do I need to loosen the clay and till it or will the top soil be enough? Should I remove all the crab grass and roots now, so my new lawn is healthy looking? Should I just cover everything with top soil and hope that grass grows in it? Please help!