To add a transition, you will want to select the icon from the left-hand side panel that says, "Open Transitions, Effects, Animations".
From here, select the "Transitions" tab. You can now apply any of the transitions by dragging the transition between the two media clips which lie on your Main Track in the timeline.
To adjust the transition, click on the transition and begin to adjust within the properties panel.
💁🏼♀️ Scenery Suggestion: Learn how to add a transition with Scenery