Start by watching the Crash Course video, Community Ecology: Feel the Love.
Competition and predation are two important ways in which organisms interact.
Competition occurs when two organisms fight for the same limited resource, and can be intraspecific (competition among members of the same species) or interspecific (competition between different species).
Predation happens when one organism captures and eats another. Take the time to watch both of the following video clips on predator-prey relationships.