Can I just click on firefox and that's all I need...I don't need to buy anything?
Yup....download and follow the installation instructions. It is free.
Also, run your sypware program virus programs. It is likely that you have at least some spyware, benign or not, on your computer. You should do this at least once a week.
bump for reference
That's a pretty broad question. As far as a browser, yes. For most, if not all purposes, firefox is all you need. The broader issue though, is that if you have a computer connected to the internet, you should get a hardware firewall. If you are a windows user, and have had the PC online for a while before installing the firewall, I'd also reccommend you reformat and reinstall your OS on your PC AFTER plugging it into the firewall.