When you drag an element of any type both to the timeline and the canvas, this element will be placed by default at the centre of the canvas. You can with the mouse pointer select and move any element all through the canvas, quickly though not in a precise way. Selecting and dragging is a helpful choice for organising quickly several elements on the canvas, but once you have located them approximately in the position they should occupy –that is, when you have established the composition of the clip–, the exact position, measured in pixels, can be adjusted from the Properties panel.
All elements offer parameters allowing the control of their exact position on the canvas. These parameters correspond to the coordinates –measured in pixels– within the canvas surface. You can choose where to place the origin of such coordinates –the so-called 0.0 point– to match whatever you want. This way, the 0.0-point can be placed on any of the four corners of the canvas or right at the centre. Based on the origin of the coordinates you choose, you will find it easier to place elements at certain positions or align different elements at a same distance from the frame.
For instance, if you want to place several elements 20 pixels below the upper edge of the canvas, you can choose the Top-left or Top-Right option and the Top field to set this distance up to 20 pixels.