Selenium is a good web application testing tool which is a free and open source software. It can run in many browsers, inmany operating systems and can controled by many programing languages and testing frameworks.
You can get more datails as well as the software from visiting the official website http://seleniumhq.org
First of all download the selenium ide from the website.
Then extrat the zip file.
Now you can see some jar file as well as some library files.
So first run the selenium server. For that you may need java. If you do not have java, you must install JDK first.
java -jar selenium-server.jar
Wait for about 1 min. It will give the configurations of your system.
Then you should deside which language you are using to control the selenium. I will use python but you can do it in come other language.(see the website)
Now you should change the directory to python library file. There you can see selenium lib files for python.
Then try to run a default test. To do that;
So now you can write your script in this folder and run it.