The key is to learn the constellations. The old fashioned-way is to get a star-map ( they appear in paper form, in books, but also online) and go out every night and , and try to identify the constellations that are in the sky, and observe how the "dome of the sky" gradually rotates during the night, with stars rising and setting. And, over the course of the year, to see how the visible constellations change. (In summer, the night side of the earth is facing different stars compared to the winter, since the earth is on the opposite side of the sun.)
Then, when you see a light that is "not supposed to be there", you know it's a planet. If it is a planet close to the sun, you will notice changes in its position relative to the surrounding stars over the course of weeks. If it is a planet farther out, you will need months to observe its progress,
On the other hand, there are apps you can download on your phone. You point your phone camera to the sky, and the screen will label everything you see!
There are websites (and monthly print magazines like "Astronomy" and "Sky and Telescope") that will tell you which planets will be visible on a given night and in which constellations.
