If you press the red button, you'll activate weapons of mass destruction and initiate a global war. As a verb, initiate means to start. As a noun, initiate (short "a") means to be part of a group.
Initiate is a tricky little devil because you pronounce the last bit one way as a verb (i-ni-she-ate) and another when it's a noun (i-in-she-at). If you're an initiate, you've been introduced to a field of study, an activity, etc. You can be a novice or a veteran. To initiate is to begin, pioneer, or broach a topic. You might say, "Let's initiate the swearing-in ceremony so our new initiates can join the Rule-Breakers' Club and still get home without missing their curfews."