This is one of the very basic skills you need to learn. Search help for setting in and out points for layers. You use the control/alt key and left and right bracket
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I second what Rick says, you really need to get a foundation first or you'll be really frustrated. Getting started with After Effects is a great place to start. It may look like an intimidating wall of text, but it helps make AE much less intimidating. Trust me - skip the basics and you'll be frustrated. This is the fastest free way to get up to speed in AE that I know of.
It is as Szalam says. Trimming is covered very early in the resources that he linked to.
Thank you all for your replies. I will definitely take a look at the link provided. Thanks again.