IMHO, it would be quicker, easier and cheaper to buy an off the shelf item than design and build your own amp. There is plenty of information on the internet about different amp chips and there are even circuits that people have built that are quite good. To make something that is efficient and that has good audio quality it would take a significant amount of research and planning. Large companies have designers that do this. I am not saying you can't achieve your goal, I am saying it will take considerably longer and the chances are that the quality won't be as good. If you also take into account the cost of components and your time, you would be well infront if you buy off the shelf.