Chess Algorithm

Question

I want to add a vs computer option in my chess game, so I'll need to make an algorithm to let the computer make a move.

How can I do this? How are the more professional chess computers build? Are they just programmed with a number of strategies? Or is it a very complicated maybe mathematical enormous piece of code?

Note: An already existing algorithm might work but if it isn't too hard I'd like to try to do it myself.