I know that is really confusing, but an example is Who’s Your Daddy. You can open cabinets by clicking on it. You can pick up/hold objects by clicking them. And there are much more examples, but for those whose played it, y’know what I mean.
You need to use a Raycast (Physics.Raycast), using values from the Camera class’s ScreenPointToRay function. The raycast function will return a RayHit, so just assign it to a variable and do
yourRaycastHit.collider.gameObject and then check whether that object can be interacted with, and if so, interact with it.
Alternatively, if You are a newcomer, and don’t feel quite at ease programing in C#, You may consider using Fungus, available also at the Asset Store (for free, MIT licence). I’m actually using it, and have only words of praise. 
It has dedicated OnDragStart and OnDragExecuted, well, let’s call them events: Fungus uses a visual coding approach, and even their examples feature a demo doing exactly what You’ve asked to do, probably more… I advise You check it out