Cut it regularly for one thing, and get yourself a copy of D. G. Hessayon's The Lawn Expert - its not very expensive and will tell you exactly what to do.

Here's something I wrote in response to a query about lawncare which should answer your question.......

Moss is caused by bad drainage and/or not enough light.

Drainage can be improved by spiking and sprinkling sharp sand into the holes. If you have trees blocking alot of the light you could consider thinning them out.

You can also buy lawn seed specially formulated for shady areas.

Moss can be killed using a granular weed and feed which includes moss killer but if you don't solve the underlying problem by improving drainage/light levels it will return

Eliminating weeds is very easy. Just water on some Verdone Extra lawn weed killer( available at Band and Q)

When you say it is rough and pitted it depends how bad it is. If it isn’t too bad you can add some garden soil to the holes and re-seed those areas.

Once you’ve got the drainage and light sorted out it is very easy to keep a lawn looking lush.

.1. Don’t cut the lawn too low( an inch is ideal) but cut it every week. If you cut it lower than an inch ( unless you have bowling green type grass species) there’s not enough leaf from the grass to grow properly. If you cut it less often the grass will snag on the mower blades and wreck the look of the grass.

2. Water on Verdone Extra to kill weeds in Mid April

3.Sprinkle on a Granular feed to make the grass grow quickly in early May

4. Water on Verdone extra in Mid July only if required to kill any other weeds)

5. Mid September scarify lawn and add an autumn feed…. this can be done with a leaf rake or depending on the size of lawn you can buy a cheap electric scarifier or hire one.

That may sound onerous but on an average sized lawn .( say 30 feet by 20 feet) all but the scarifying shouldn’t take more than 10 minutes per operation..

Once you’ve scarified you can probably get away with doing that 2 yearly.

The above method assumes you’re prepared to use weed killers/fertilisers. If you’re not you’ll need to remove the weeds manually which is a big job.

Perhaps you could compromise and use weed killers to eliminate the existing weeds and after that remove the few that reappear manually

Excellent lawn care site here

http://www.lawnsmith.co.uk/lawn-care-advice/lawn-faqs/weed-control/

Well two very good replies,
But keep in mind what sort of lawn you want,
ie if you've children playing on the lawn reguarly ie football etc or like myself 4x dogs running about on it then you'd be better having a grassed area to fit this sort of treatment,
But no matter what you want you'll need to keep it cared for and the first step is cutting it and not letting it grow more than 2 inch high, most of the larger weeds cant live if they cant grow seed heads and by regular cutting this wont happen, the smaller weeds can be delt with via treatment.