How to know when to preform burn?

Switch to map view, and select the moon or planet. Then click the little icon on the right under "Map View" that looks like four corners. This sets that body as your target. Then you'll see a green and orange arrow (nodes) appear along that body's orbit indicating alignment to your periapsis and apoapsis. Warp ahead until that arrow starts to come around to the body. (There's actually a button that will warp you directly to that node.) If you select "High" under UI Verbosity, it will give you a countdown. At 0, fire your engines. As your energy builds, your orbit will begin to grow more parabolic, with the periapsis approaching the target body's orbit. The UI will then give you a bunch of indicators about your approach, intercept, etc.

You can also do a planned burn: https://www.simplerockets.com/Blog/View/1919/Planned-Burns

https://www.jpl.nasa.gov/edu/teach/activity/lets-go-to-mars-calculating-launch-windows/