How do I fix my pc keyboard?

Desktop or laptop model?

Did you spill ANY liquid on the keyboard? Rubbing alchool should not cause any problem, but any other liquid definitely can ruin your keyboard.

For a desktop type, replacing the keyboard is very easy and relatively cheap. Does using a replacement keyboard solve your problem?
On a laptop, try connecting an USB keyboard. Does it work?

You may have something that intercept the key strokes and don't properly forward them. It may be an utility/accessory application or a buggy malware.

Install and run some anti-virus. (some very good free offerings are AVG free, Avira free version, Avast free version)
Install and run some anti-spyware. (one of the best is Spybot)

If you use some utility that intercept keystrokes, you may need to reinstall it.

Electro
July 2014