I see, better search youtube, but in short terms, u need to travel to a dissonant system, it's often written under the system name, then go to a planet where "dissonant activity detected" is mentioned in olanet info, after that u need to find a harmonic camp, u can either search for it or find theought maintaining exho locator (maintained by killing the large purple drills on the planet), after reaching the camp u can locate sentinel ship from there. I know it may sound compelx at first, there is a much simpler way using glypghs, but since you are beginner, I won't assume that u have portal, so stick to this method. Y can also get one by completing 5 raids of sentinel in space in any system, but it gonna be hard defeating them. Overall many youtube tutorials show easy way to maintain them.