The Website : http://livesplit.github.io/downloads/

Click on LiveSplit 1.7.6 (Maybe there is a newer version of it already) then the program should start downloading.

Then install it and start "LiveSplit.exe"

You can start the timer with "F1" on your Keyboard

You can also create Splits for longer Speedruns, to insert Splits right click on the timer and click "Edit Splits". Then you can insert your Splits and other details if you want to.