Welcome to the forum!
Download an anti virus program, if you haven't already. Some free and good anti virus:
AVG, Avira, Avast, Window security Essential
I personally use Avira and to turn it on, you click on the big obvious "activate" on the graphic user interface. I used to use AVG and if I remember correctly, the turn on option is also quite obvious. Look for it, it should be easy to find.