So, first of all,
disneyland resort is composed of two theme parks, disneyland park and california adventure, a shopping and dining district, downtown disney, three onsite hotels, and several other hotels located offsite. California adventure is a theme park representing aspects of california culture. Some of it's headliners include soarin' over california, radiator springs racers, and tower of terror. Disneyland park is a place where tomorrow, fantasy, and yesterday collide. It has a family friendly ambiance and is the only disney park where Walt actually walked.
As for the best time to go, the months january, february, may, september, and november are the least crowded, but they also have the most ride refurbs. This year, disney is celebrating it's 60th anniversay, so the celebration will bring larger than normal crowds.The celebration will last all summer, but they haven't released much info on it yet. From january until may, there will be larger than normal refurb activity to prepare for the celebration, so I do not recommend going around that time, as you will not get to experience DLR at its best. Also, avoid the two weeks around christmas and new years, as that is the absolute busiest time of the year.
For transportation, the john wayne airport (SNA) s the closest and most convenient and easy to get through, but the los angeles airport is usually the cheapest and has the most flights available.
You cannot make dining reservations unil 60 days in advance. But usually, reservations are still available even a couple of days I advance. Most of the times, you can just walk up and get seated, if it is low-season.
I recommend you start with disneyland.com, disneyland's official website, there is a wealth of information on there, including pictures, advice, and detailed descriptions of the attracions and venues.
4 or 5 days is a perfect amount of time to spend at disneyland! It is such a magical place, and you are bound to have a good time at the happiest place on earth!