In the top left of your screen you will find a 'drop down menu'. When you open this menu you will find the galaxy locator. Use the locator by highlighting the planet you're looking for and clicking the 'locate now' button.
Now you know in which system the planet is located. To find your way to this system you can use the galaxy map, located in the bottom right of your screen (an icon which looks like a white card with blue and green dots on it).
Clicking the icon will open a new screen containing a galaxy map, which is probably bigger than your screen. So you need to navigate trough it by holding down a left mouseclick while you move the mouse around. Wait for the map to load on first use and it will jump to your current location when loaded.