New Lawn Problems - Please Help!

Hello all! I now have a garden, and have no gardening experience. First thing I want to do is create a nice flat lawn area in my garden. As you can see from the pictures the lawn is very patchy, I found out a lot of the garden actually had concrete about 2 inches below the surface, which I have dug up, which now leaves me with a very uneven and patchy garden. 
I'm not sure where to start, any help would be greatly appreciated!