The Sentry Bot is an extremely overpowered “death ray machine” that destroys any enemy in sight, though for some reason this Sentry Bot is available straight away after starting the game.
First, you must go to the Robotics Disposal Ground, it is just right of the Sanctuary . It is located here:
Upon entering, enter the compound’s shelter. Once you have entered the building go to the terminal in front of you.
Once you are in the Terminal Eject to Holotape and exit the Terminal. Then make your way to the inactive sentry bot in the junkyard.
Note: To eject the terminal you must go to the terminals home page and click the assigned key/button
Open your Pip-Boy and go to the Misc tab. From here find the Combat Sentry Proto MKIV Holotape and play it. You will now have the option to do many things with the Sentry Bot such as to Self Destruct and to move around.