To be certain that you won't have a weed problem with the lawn in the future, it is best to treat the whole area well with roundup and then treat it again after 2 weeks. Then as Fred says, after a week or so dig it over, rake it and flatten it down.
After that it's up to you whether you buy lawn turf, lay it and water it in, or whether you use grass seed which in the UK takes a lot longer to turn into a nice lawn.
Iain.