POAPs represent the bookmarks of your life. They are proof that you attended an event (POAP = Proof of Attendance Protocol). To collect POAPs, you just need to attend an event!
New drops are best discovered within the communities in which you take part. However, if you'd like to see a list of upcoming drops, you can always check the POAP events page. Keep in mind that private drops will not show up there.
Issuers who have set up these POAP drops have the option to include information within the description as to how one might go about collecting their POAP. Sometimes you will even find links directing you to their website or meeting page in order to take part in their event.
It's generally considered bad practice to attend an event solely for the purpose of collecting a POAP. POAPs are meant to enhance and enrich the experience of taking part in an event and are not intended to be hunted down by individuals with no interest in the event itself. Please be a responsible POAP collector!