Jam, you currently use 100% of your brain. We all do. The beautiful thing about plasticity is that it means your brain isn't static. It can grow and shrink in size and neural density. The day to start growing your brain is now. As we get older, our brain tends to become more 'fixed', more static. In fact, without stimulation, it begins to shrivel. We must exercise our brain, daily if possible.
Also, avoid stress, depression, anxiety, or anything that is mentally and emotionally draining.