Strategy is defined in the Compact Oxford English Dictionary as "1. a plan designed to achieve a particular long-term aim. 2. the art of planning and directing military activity in a war or battle".
As the Supreme Naval Commander you make the strategic decisions: where to direct your resources and, what sort of navy, airforce and troops you want that will best achieve victory.
You also make the grand tactical decisions -deploying your ships in fleets and giving them operational orders,as well as deploying aircraft and troops for defensive or offensive operations.
To help you, you have a 2-I-C who will follow one of four pre-defined strategies -very cautious, cautious, aggressive or very aggressive.
Each of these strategies has a favoured approach to the kind of resources you build and how they are deployed.
Follow these links for more information:
You can change your strategy at any time, but first you should be aware of the need to avoid unwanted effects from too much change. Follow the links above to understand what the effects would be of a change in strategy.