Getting better at racing the AI in CSR Racing 2 is a pretty straightforward process. You run enough races with a particular car to get its shift timing down, continually improve it by buying upgrades whenever you can, and add any fusion parts you grab along the way.
Tuning adds another dimension to the whole affair, but not enough to significantly throw you off course. If you just keep plowing ahead, you'll be able to take down all the crews in the game eventually and figure out what happened to your friends' stolen cars.
The Live Race mode, however, is a different beast. In contrast to some other mobile racing games, it provides CSR Racing 2 with a true multiplayer experience, allowing you to race head to head with a live human opponent at any time (or at least any time the servers are functioning properly).