One thing is to evolve it into Raichu, who has much better stats overall.
The main way to improve stats is to EV-train. You can read our page for more details: http://pokemondb.net/ev
Since your Pikachu is high level already, you will need to get rid of the EVs you already have (they will be maxed out by now). You do this by feeding it special berries (see link).
However, Pikachu/Raichu's strength is not defensive, so you are better off training in Speed and either Attack or Special Attack, the idea being you can take down many opponents quickly and in one hit, before they get a chance to hit you. If you don't evolve Pikachu, get it to hold a Light Ball to double its Atk/SpAtk.