Disneyland is the happiest place on earth but it can be a nightmare if you come at the wrong time. To help save you a lot of time I will offer my decades of living close to Anaheim and all those years of having season passes to help you have a better trip to the happiest place on earth.
One of the first things that you should decide is when you want to visit the park. If possible the best time to visit is the winter or fall during the school week when people are in school. If you have to go during the summer then make sure to go early in the week the later in the week it gets the worse the crowds get. I hardly even visit the park in the summer except to stop by if it’s on my way for a quick late night ride or to get something in one of the stores.
If you do have to go during the summer the good news is that Disney tries to make up for the crowds by offering you more attractions and opening the park longer. Unfortunately though you are better off having a much shorter stay then a longer more crowded one. The other bright side is that using your fast pass will help you get on more attractions then you would have during the summer before fast passes were introduced.
It is a good idea to go online to Disney’s official site to get any information before hand such as ticket prices, park operating hours, and if any rides are closed for refurbishment. While online check the map of the park and get an idea of what your ride priorities are.
When you arrive be sure to get to the park early, you need to allow time to park, get on the tram, buy tickets, get through security, etc. Another important factor in getting early is that while the park opens at the time they post they allow you into the park typically about an hour before the park opens they only allow you into Main Street but getting to the front of Main Street and waiting for the ropes to be dropped to let you into the rest of the park gives you a huge head start on the guy who thought he needed to be there at 9 and is still in the line to get in the park or even getting his tickets. The parks first couple of hours are some of the choicest ride times the latter it gets the worse the lines will get. Caveat emptor the rope drop can be very competitive you really should be in good shape to run and dodge the kamikaze strollers.
I would advise you to plan where to see first based on what you are most interested in. If you really want to go to Pirates of the Caribbean the Haunted Mansion, and Space Mountain then you might want to head the Haunted Mansion first and get Pirates done quickly too. However I think that the Space Mountain side has more popular rides closely condensed together such as Buzz Lightyear, Space Mountain, the Submarines, and Star Tours just right there in Tomorrowland with the Materhorn and Toontown not far. One warning I will give that you probably wont get anywhere else is that even in the slowest months Fantasyland is packed until late so if there are a few favorites this might be the way to go on a recent trip on the first Monday in December I never waited more then 20 minutes for a ride (Space Mountain and Splash Mountain both that long at longest point) but when I went on the teacups I found myself waiting over 30 minutes and it was probably one of the least crowded Fantasyland attractions at the time.