Rather than a lengthy explanation, I suggest you beg, steal or borrow a copy of D. G. Hessayon's Lawn Expert - its only a tenner new, often the library has copies. or you can find it on Amazon second hand sometimes. Tells you everything you ever wanted to know about lawns and more.

You dont say you have problems? So in a nut shell! rake it over now (use a lawn scarifying rake) to remove old dead grass, feed-n-weed now and again in 6 weeks and regular mowing to encourage side way growth. Trim the edges now - will last a good while. Dont need to water in high summer as it will recover when it rains. I agree with Bamboo those series of books are great. Just make sure you get the latest edition when you buy!