Hi.  The answer to your question concerning affordable anti-virus software is very simple.  Abstinence is generally the most affordable way to go in this situation.  I know this may sound a bit harsh, but you are still young and computers can wait until you are older and more mature.  With computers comes a lot of responsibilities.  Lots of people your age think that a virus is something that won't happen to you, but let me tell you something sister, it can.  It just takes one night with the wrong website and you will have to live with the consequences for the rest of your life.  Think about that before you get a computer. 

Hi.  I'm the TechGuy.  I'm here to answer all of your techmological questions about techmical things.  I know pretty much all there is to know about techmology and jargon.  Let me know if you need any questions answered.  Just send me a message and I will post the reply here. 

~TechGuy