The sooner you read this informative article of Tim Bowen's, the better.
As the comparative form of soon, sooner simply means ‘earlier than expected’, as in ‘My prediction has come true sooner than I thought' or ‘The flight arrived sooner than we expected’. It also has some other uses, notably as a synonym for rather to indicate preference, as in 'I'd sooner she married no-one than marry a fool like him' or 'I'd sooner stay in this evening than go out for a meal'.
With no and than, it is also used for saying that something happens immediately after something else, as in ‘No sooner had I walked in the door than the telephone rang’ or ‘No sooner had we got to the station than the train came in’. Note the inversion after no sooner and the use of the past perfect tense for the first action.
Sooner is also used in the expression the sooner the better to mean ‘as soon as possible', as in ‘You’ll have to face her, and the sooner the better’. The sooner … the sooner is used for saying that you want something to happen soon so that something else can also happen, as in 'The sooner you do your homework, the sooner you can go out'. You can use the phrase sooner or later to indicate that something will definitely happen at some stage in the future even if you do not know exactly when, as in ‘The whole thing is going to reach crisis point sooner or later’. The fixed expression no sooner said than done is used as a response to a request if you want to indicate that you will do it immediately, e.g. ‘Could you pass me the salt?' ,'No sooner said than done!'