<font color=navy>To add just a little to all that has been said so far ...
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If this picture works...
The park at the top is Disneyland. Tomorrowland is to the right of the Matterhorn Mtn., and that body of water is the submarine lagoon. The monorail station is next to that lagoon.
The park at the bottom is California Adventure, and the monorail goes over the "Golden Gate Bridge," but it does not stop at DCA. To the left of the bridge is the building for Soaring Over California, and to the left of that is the Grand Californian Hotel.
The Paradise Pier Hotel is at the bottom left, with the pool, and at the far left is the Disneyland Hotel.
Going from the Disneyland Hotel, the building with the green roof (is that a baseball diamond?) is the ESPN Zone, and the brown building to the right (with water on the left side) is the Rainforest Cafe. The monorail station would be to the right of that - you can kind of see the roof of the station where it's drawn in. Across from Rainforest Cafe w/the squiggly yellow lines is the AMC theater, etc.
Downtown Disney stretches from the ESPN Zone all the way to the end building of the Grand Californian, and about where Soaring Over California is. You can see the Plaza between DL and DCA.
Okay, If you are at the Grand Californian (GCH), then your best bet is to go through the Downtown Disney entrance and walk to the main entrance of Disneyland. You can also use the private entrance to go into DCA (through the building to the right of the pool).
If you are at the Disneyland Hotel (DLH), then you can either walk through Downtown Disney, or you can walk to the monorail entrance next to the Rainforest Cafe (RFC). The advantage of using the Downtown Disney station is that the line is usually shorter, which means that you won't have to wait so long to go through security, and you save that walk to the main entrance. If you are a DLR hotel guest, you can show your room key for 'front of the line' - or I should say, Hotel designated line. If you are staying at the Bonita Tower of the DLH then your best options are to use the monorail, or cross the street, and go through the GCH lobby to Downtown Disney & DLR - you'll save a lot of walking this way. The disadvantage of using the monorail would be if they aren't using all of their trains, you may have to wait for the next one. Sometimes it's quicker just to walk than to use the monorail
If you are staying at the Paradise Pier Hotel (PPH), then you have the same options as the Bonita Tower at the DLH. The additional benefit, is that you have a separate entrance to DCA, which is located just below the Grand Californian, by the orange squiggly line (Mulholland Madness rollercoaster ride).
I recommend that for the first time you enter either park, go through the main entrance to get the full effect.