Selenium has a few different components that you should look at just
so that you know what you're getting yourself into.
Selenium-IDE will allow you to record and playback basic tests in
Firefox. I personally don't use Selenium IDE for anything.
To do more complex automation tasks, you will need to use a
programming language. You get to pick from a few different
programming languages. If you go this route, you will need to know
that programming language. If you're coming from QTP, Java or C# will
probably be the best bet for you. Once you pick your programming
language, you can download the Selenium Client Drivers for your
specific language. I picked Java, therefore I downloaded the Java
Client Drivers. Those Client Drivers are essentially libraries that
you will import or use in your code. If you go this route, you will
probably want a programming environment (aka IDE), such as Visual
Studio, Eclipse, etc.
Once you do a bit more research on Selenium, come back to this forum
and post follow up questions as needed....
Good luck!