How to draw microstrip line in EAGLE?
Microstrip simply means a controlled-impedance trace on one of the surface layers. A stripline is the same, except on an internal layer. 



A Microstrip is just a regular track but you determine its required width in order to have a specific characteristic impedance (50 ohms in your case) based on the copper weight and distance between it and the ground plane beneath it on an internal layer (or bottom layer, if you only have a two-layer board). This is one of the most accurate online calculators I know of that will help you determine the correct width of your trace: https://www.eeweb.com/tools/microstrip-impedance