weeds in new lawn.

i have just sown a new lawn 2 months ago,it is very patchy with lots of weeds like chickweed mixed through it.thistles are also a problem. i have cut it every week with the hope of killing off the weeds but they still remain.in the patchy areas i have applied more seed but the lawn looks very dirty.what can i do to rectify this?lots of people are telling me to give it a weed and feed, but is the lawn not too young?