Sometimes you need to see the big picture of your entire timeline, other times you zoom to hunt for a single frame.
Steps to zoom and navigate the timeline
At the bottom of the timeline, drag the bar left or right to zoom in or out.

Hold Alt (Windows) / Option (Mac) and scroll your mouse wheel over the timeline. It is way faster than dragging the bar once you get used to it.
To move around the timeline, drag the horizontal scrollbar at the bottom or hold spacebar and drag the timeline with your mouse.

One more thing
Press + or – on your keyboard to zoom in and out instantly. No mouse needed.
Related how-tos
How to use the timeline in After Effects
How to use the toolbar in After Effects
How to add a new solid layer in After Effects
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