From the moment you enter Disneyland, there's entertainment around every corner. Even after visiting the parks dozens of times, I always start by picking up a time guide at the turnstile. This guide lists most of the live entertainment throughout the park and each printed guide usually covers 3-7 days. The schedules for some shows change day-to-day and others are totally consistent, so that guide is key to getting around on time.
I tend to break up Disneyland entertainment into a few categories: Seasonal, Casual, Ambience, Characters and Shows.
Seasonal Entertainment encompasses, of course, Christmas and Halloween fireworks and parades, but Disneyland also takes advantage of other times of year like summer, holidays like Veterans Day and movie blockbusters as an opportunity for a new pre-parade, fireworks show, street party, or musical tribute.
You may very well pass by some Casual Entertainment without even realizing it was a scheduled event. The best example is the Dapper Dans - a barbershop quartet in snazzy striped suits with matching hats that sing up and down Main Street, as well as occasionally riding the Horse Drawn Carriage or Firetruck. Just like the Dans, the Disneyland Band, Army Men and Pearly Band tend to march around and can be unpredictable in their start and end location.
Throughout the lands of both parks, Ambience Entertainment happens all around you, adding to the Disney Magic. Custodians that paint character's faces on the sidewalk with brooms and water, Citizens of Buena Vista and their pets will carry on with you in-character, and The Laughing Stock (in front of the Golden Horseshoe) all can be found through the time guide, but don't worry too much about scheduling your time around them - part of the fun is that they seem to appear out of nowhere.
Disney Characters have their own section in the time guide although the times listed are vague at best. Most characters are available for photos and autographs for about 30-45 minutes and then take a break for about 10-20. If you're looking for a specific character, Cast Members can point you to the area that character hangs out (Peter Pan, for example, is usually found around the Mad Tea Party teacups in Fantasyland) but sometimes, it's just a waiting game.
Be sure to always keep your eyes open for unscheduled character appearances - you never know who might show up.
Shows are the most scheduled entertainment and require the most pre-planning.
Some shows are nearly always easy to get in to and are worth the short wait. Mickey and the Magical Map just requires showing up about 30 minutes early to ensure you get a seat. Some shows have smaller seating areas, so you need to keep an eye out for a quickly growing line, like Disney Jr in DCA and Fantasy Faire at the Royal Theatre. Frozen - Live at the Hyperion, however, isn't worth the wait, there are tons of stairs to climb, and because of the lack of enthusiasm for the show, they're limiting the number of shows per day, so every show is packed. If you feel the burning desire to see Frozen, make sure you get in line about an hour before showtime.
The most popular shows like Fantasmic! and World of Color require a FastPass to determine your entry location and standing area. If you're planning on catching one of these shows (especially if you'd prefer the first show of the night) be sure to get those FastPasses within the first couple hours of your day.
The Fireworks are the ultimate cap to a Disneyland Vacation. Even though the show may change from summer to winter and from year to year, they all are the best fireworks you will ever see. Everyone wants to watch the fireworks from Main Street, so space starts filling up hours ahead of showtime. Disneyland recommends Small World and the Rivers of America as alternate viewing locations, but in reality, you can enjoy the fireworks from almost everywhere in the park (and even the esplanade between parks) and most locations - besides Main Street - have a more manageable crowd.