Rest assured that your capacity to acquire new skills and knowledge is massive.
You don't just get smarter. You get smarter at something in particular. Playing chess, doing IQ tests, running the 100m dash, programming, social skills, public speaking, etc. So you need to pick a particular skill or set of skills or vocation and decide to get smarter at that.
There are some general rules for improving brain function though. Here are a few:
1. Read books. Reading trains your brain to concentrate for long periods of time without fatigue or distraction. There is a growing school of thought that the short bursts of reading and frequent distractions we experience online are harming our ability for deep contemplation, introspection and concentration. See Nicholas Carr, The Shallows. http://n.pr/bnAfRV
2. Try to get 10 hours of sleep a night. Sleep improves mental and athletic performance. http://n.pr/9wQsXr
3. Maintain your cardiovascular fitness. I highly recommend running. After years of cycling, swimming, hiking, etc I've found that running gives my brain function the biggest boost and provides me with sustained mental energy through the day. A good cardiovascular system supplies your brain with plenty of healthy oxygen rich blood. It's like putting racing fuel in your car.
4. Eat well. Cook your own food. Avoid processed or pre-prepared foods and non-organic foods (mainly due to the pesticides). Fish is awesome, but watch out for mercury.
5. Don't drink anything stronger than wine. Don't do drugs. (just like your mom told you)
6. Watch your weight. I find the biggest source of mental fatigue is when I've gained a few pounds.
Good luck, and congratulations on making the decision at a relatively young age to focus on your mental fitness.