Add seconds to beginning of timeline.

Go to the Composition Settings and increase your Composition Duration by two seconds.

Back in the Timeline, Zoom Out to see the additinoal two seconds (click and drag the right Zoom Handle to the right or press "-" on the Standard Keyboard).

Go to two seconds (0:00:02:00), select all (command a, mac; or control a, Windows) and then click and drag the selected Layer Time Spans such that In Point of the left most Layer Time Span aligns with the Current Time Indicator at two seconds.

You now have a two second gap at the head of the Composition Timeline.

If this is the first time you've made this kind of edit and are worried that you may not do it correctly, select your Composition in the Project tab and choose Edit > Duplicate to create a copy.  If adding two seconds to the Comp works as expected, you can delete the duplicate.  If things don't work as expected, you have the original and can try again after making another duplicate.

I'm sure you can find soem script at AEScripts.com that shifts your layers forward in time and extends the comp. Otherwise you can of course do it manually by changing the comp settings and dragging your layers around. In fact if nothing else is required, you could simply pre-compose your existin work and move the resulting layer as a whole in the parent comp. Anyway, you may want to read the online help on some basics.

https://helpx.adobe.com/after-effects/using/composition-basics.html

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