Well, first of all, change the attitude. If you want to start heading in a positive direction you need to want to feel/be better.
Start thinking smart, and you'll start being smart.
Keep stimulizing your brain. As you get older your brain becomes harder and harder to stimulize, hence the reason why after you hit your twenties you start to lose brain cells a few at a time.
Try taking some online memory games, and start emercing yourself into the aspect of learning new things.
Read, read, read. Keeping up with what's new, or what the classics can teach will not only help raise your IQ and give way to more in depth thought, but itll make you a much more charming person to be around. The more knowledge you gain, the more you have to offer to the table.
If you dont already, try to start getting a good nights rest. Sleep is key to not only learning, but retaining what you have learned and staying positive as well as motivated.
Create a healthy diet for yourself. A healthy diet (mixed with good rest) will help give you that push to face the world in a more positive manner.