Lets say you wan to find out the id of the opportunity from a visualforce page:

public class YourClass {

    public string OppId{get;set;} // To capture Opportunity Record Id (Dynamic)

    // constructor
    public YourClass (apexpages.StandardController sc) {
        OppId=apexpages.currentPage().getparameters().get('id'); 
        system.debug('opportunity id'+ OppId);
    }
}